Below are my opinionated but hopefully useful thoughts on selecting a rear support bracket and shortening the driveshaft.
The Rear Crossmember
There are basically four ways to support the rear of the 5-speed: 1) Weld or drill-and-bolt on a new set of factory tabs, 2) drill-and-bolt on a commercially-available rear brace, 3) use a no-weld no-drill-and-bolt U-shaped bracket, or 4) roll your own.
As per Ben Thongsai's bible (http://www.bimmers.com/02/upgrades/transmission.html), you can buy the factory transmission support brackets (part number 41 12 1 808 830), new, for about $15 a side, relocate them behind the original ones and 80mm up from the bottom of the frame rails (this is how it's described in the blue BMW 2002 factory manual), weld or bolt them to the transmission tunnel, and simply shorten your existing rear cross-piece to accommodate the fact that the tunnel is narrower further back. This is mechanically stable, since it supports the back of the transmission by locating the brace and tabs right there, at the back of the transmission. And it sounds easy and cheap. On paper, it is the least expensive option. But I tried it, and getting the bracket locations correct was more difficult than I thought, and my skills using a wire-feed welder to hold them in place were laughably poor, so I wound up having to drill holes in the transmission tunnel and bolt them in place. Plus, the old brackets are still there. I don't care, but if you have a concours car, you might. If you or your shop's welding and grinding skills are good, you could probably remove the old brackets, grind and touch up the spot where they were, weld on the new brackets, paint, and have the whole thing look factory. And that's how you turn the least expensive option into the most expensive option.
The next option is to use a commercially-available bracket/cross-piece that is meant to be bolted to the tunnel at the back of the transmission. One is the Ireland Engineering rear cross member ($110, http://www.iemotorsport.com/mm5/merchant.mvc?Screen=PROD&Store_Code=ie&Product_Code=025spcross&Category_Code=2002-clutch-driveline), which is basically the same in form and function as the factory brackets and a cut cross-piece. The other is the Top End Performance rear crossmember ($110, http://www.racetep.com/5speed.html) that differs from the Ireland product in that it attaches to the original brackets AND ALSO bolts to the tunnel. I haven't used either the Ireland or TEP products, but it's my assumption that the TEP piece is probably self-measuring. That is, I assume that you bolt it to the transmission and to the existing support brackets, then mark the location of the rear holes, and drill there. In contrast, I assume that the fore-and-aft and height of the Ireland piece needs to be measured, just like using new factory tabs.
The third option is to use one of the methods that do not require relocating the support brackets or drilling holes in the tunnel for new ones. There are two that I am aware of. One is the U-shaped bracket pioneered by Rob Torres at 2002Haus. 2002Haus does not appear to be selling these currently on their web site. If you're interested, I encourage you to contact Rob Torres directly (he's a great guy). For many years, Rob did not sell the U-shaped bracket a la carte; he only sold it as part of his 2002 5-speed kit. Because of this, for better or worse, some people copied the U-shaped design and began selling a similar bracket ($45, http://www.classicdaily.net/products-page/bmw/bmw-2002-5-speed-bracket/). As I said, the advantage of these brackets is that they go right in without drilling or measuring. The disadvantage is that they do not support the weight of the transmission at the back; they support it by cantilevering the load on the original mounting tabs. Some folks have reported that, on cars that are tracked or aggressively driven, the tabs have bent or broken from the cantilevered load. My personal opinion is that, for a lightly-driven car, it's fine. I've used both the 2002Haus bracket, which went in with flawlessly with no modification whatsoever, and a copy (I can't remember if it was from ClassicDaily or just some guy on eBay), which required me to elongate the holes in the bracket for both the mounting tab bolts and the rubber mount, as they did not line up with the pieces in my car. Still, a trivial installation. Drilling holes in 2002s gives me the heebie jeebies, especially if there's an acceptable alternative.
The other no-drill bracket is from Metric Mechanic. They no longer show it on their web site, but I asked Metric about it a few years ago and they still sell it, though it was expensive. There are photos of it here (http://www.bmw2002faq.com/topic/93371-seeking-5-speed-conversion-information/page-2). It attaches to both transmission tabs and has adjustment built into it via a clever sliding mechanism.
Lastly, you can roll your own. Look through the FAQ and you'll see things put together with angle iron. When a U-shaped bracket is $45, I don't see the point.
Keep in mind that, even with the U-shaped brackets that I say "do not require drilling or measuring," ANY of these methods will almost certainly require final adjustment to get the guibo aligned with the face of the rear flange. This generally involves using washers to raise or lower the back of the transmission or the center support bearing, and will be the subject of a later post.
So, which should you use? If you're not going to be happy unless the underside of your car looks factory-stock (except, I guess, for the Getrag 245 and all those other conversion parts :^), go with option 1 with new factory tabs welded on, the original tabs removed, and everything cleaned up and painted all pretty. If you track or beat the crap out of your car and are concerned that supporting the trani from the original mounting tabs isn't going to be strong enough, then use option 1 or 2 with a bracket that goes straight across the back but requires welding or drilling. But if your car is lightly-driven and you want something quick, easy, and inexpensive, then use option 3, the U-shaped bracket.
It is well-documented that the driveshaft must be shortened by the difference in length between the 4-speed and the 5-speed -- 3.5625" -- then balanced. If you look on the Top End Performance web site, they rant that anyone who uses this canned number rather than fitting the 5-speed and measuring the flange-to-flange length from differential to transmission on their car doesn't know what they're doing because each car is different. I've tried measuring it myself, and due to questions on exactly what constitutes the flange face, sag of the tape measure, inaccuracy of the first 1/4 of the tape measure due to motion of the little right-angle tab, and other things, I KNOW that I can't measure that distance to within 1/8". Hell, I've tried measuring a driveshaft laying on the ground, and due to the same issues, plus lack of parallelism between the front and rear flanges (they're on universal joints), I know I can't do it accurately. Thus, my personal opinion is that the danger you'll get the measurement wrong is greater than the danger your car has a differential-to-transmission distance grossly different from other 2002s.
Plus, there's a secret to this.
Are you ready?
The differential location is adjustable fore-to-aft by almost 1/2". Undo the six bolts holding up the differential. Go ahead. It's not going to fall out; it's still sitting in the carrier. The holes those bolts go in are oblong.
So here's what you do. You get your driveshaft shortened by a reputable shop by the magic, correct 3.5625". You loosen those six differential bolts. You bolt the guibo to the driveshaft and test-install the shaft, bolting the rear of the shaft to the differential, putting the snout of the driveshaft in the rear transmission flange, but leaving the center support bearing nuts loose. Then you push forward, hard, on the back of the differential (I position a block of wood on the beefy part of the case, then tap it forward with a small sledge). Then you look at the space between the front face of the guibo and the rear face of the transmission flange. If some ears on the flange are touching the guibo, you're good, and the only remaining issue is the driveshaft-to-guibo alignment that any 5-speed installation needs to deal with. But if NONE of the ears are touching the guibo, the differential and driveshaft have to come further forward. If the diff is all the way forward in its oval slots and there's still space, you can try pulling the engine back on its mounts a bit.
Once I've pushed the differential forward and verified that the guibo and flange touch, I put in the four bolts attaching the guibo to the flange, tighten them (but not torque them down) so the driveshaft and rear end are pulled forward, THEN tighten the six differential bolts, THEN take the four guibo bolts back out and check for guibo-to-flange alignment.
Before I leave the driveshaft, a word on used versus rebuilt versus grease-able. The universal joints in BMW driveshafts are long-lived but are not easily rebuild-able by the do-it-yourselfer. Further, they do not have grease nipples. When you need to have a shaft shortened, you can a) simply have your existing shaft shortened, buy an exchange shortened shaft, c) buy a shortened shaft in which the universals have been rebuilt using the original-style non-grease-able joints, or d) buy a shortened shaft built with joints with grease fittings. Obviously the costs increase as you move up this chain of options. No value judgement here, but after owning 25 2002s, including ones that have lived hard New England daily driver lives, I've only ever swapped a driveshaft once because its universals were bad or noisy. If I'm having a shaft shortened, I just hand-rotate the front and rear universals, and if they're smooth, I don't worry about it, but if you're restoring a car from the ground up and want more, I understand.
(next: enough talk! so remove and install the damned thing already!)