1977-79 320i Alternator. These can be bought new or found at the junkyard. You might want to get new alternator bushings at the same time. Most recommend urethane bushings as they will outlast the stock rubber ones by a considerable margin.

1. Disconnect Battery
2. Remove stock 45 AMP Alternator - Remove 3 prong wiring harness and the 10mm bolt that holds the red wire. Remove bottom 13mm nut and bolt from the tensioning arm. Remove the top 13mm bolt. Remove Alternator, and list it on fleaBay (RARE! Original! BMW Alternator! ;)).
3. Remove the square voltage regulator located next to the relays on the drive side engine compartment.
4. The 320i alternator is internally regulated, so 2 wires only need to be hooked up. The wiring is very easy. The thick red wire goes back on the 320i alternator, on the "B+" post - this wire does the charging. Cut the BLUE wire off the stock 3 prong harness and connect it to the "D+" terminal on the alternator.
5. Hook up the ground wire (brown) just like it was on your stock alternator
6. Bolt up 320i alternator in the reverse of the removal procedure of the stock one.
7. Tighten the fan belt just enough so that you can take the belt between the water pump pulley and the alternator pulley and twist it 180deg. Don't overtighten it because you will kill the water pump bearings. If it squeaks under load (when lights, etc., are on), snug it up a little more.
8. Enjoy brighter lights and fewer charging problems! Take her for a night drive! :)