You’ll never know how much rust you have till you take all the old insulation out. A surprising amount of water collects over the years around the heater box, pedal box and floor plugs. Dry ice is best way to break up the old stuff. Sometimes helps to use cardboard or styrofoam baffles to hold the mush in place on vertical surfaces like trans tunnel. No metal implements needed (and why brook more rust by compromising paint where it’s still good?). You’ll go through a few plastic putty knives, but it all pops off eventually. Residue removal using acetone and soft brushes/cloths. POR self-etching primer/POR-15 to reseal the surface.
Remove and treat the floor plugs, which I assure you are not sealing even if they appear tight. Then replace them using plenty of seam sealer or other suitable sealer.
Remove and re-seal the heater box — in fact, I suggest you rebuild it or have Hami Pramono do so. If you have AC, you never want to have to service the heater box. Not hard to get out and out if you have the console out.
I looked at a lot of products for vibration dampening. Settled on SecondSkin, which is high quality, easy to apply, won’t smell or fume. It’s basically the same as dynamat, adhesive/rubber/foil, but a touch less expensive. For the serious soundproofing layer I used SS’s deluxe butyl layer, over 1/2” thick, and heavy. I cut and taped together and laid down a single sheet on the floor on each side of the tunnel, from under the dash to the rear seat. Then taped together pieces (it does best if it’s contiguous) to cover the rear floor/trunk surfaces. Laid loose strips in the bottoms of the doors and rear quarter panels to pick up sound there. The greatest thing: didn’t have to use any glue or other adhesive, and the thick layer will not bunch up under the carpet. When I need to get under it I can just lift the carpet and then the butyl layer. And it EATS sound.
Everywhere else: trans tunnel, quarterpanel walls, inner door skins, I used Siless 1/8” closed cell foam, making sure to leave weep holes at places like trim and ensuring no water catchments exist at top edges (the adhesive is good, but it’s tough to stick it down in some of the recesses). I doubled it in some places, but left a single layer on the trans tunnel and Esty’s beautiful #405 carpet fit over it like a glove. Between the SS dampener, the Siless foam and the SS butyl layer, with the Koogle underdash panels in place the car is pretty quiet. Mind you, have not tried my hands-free phone yet.