the turbo cars came with 3.90 gears for the 88 year, regardless of auto or 5spd. the non turbos came with 3.73 (4spd) and 4.10 (auto).
http://www.willdabeast.net/Supra.html at the bottom is a chart that will decypher you axle code. this code will be on the engine bay tag
on my red 88 i had when i was in calgary i was running 10-11 psi on the stock fuel system and it handled fine. once i past the 12 psi mark i hit fuel cut. that was with stock exaust installed, with the 3 inch it did ran at 15psi till i layed off the gas and blew the intake hose from turbo to i/c, hose split right in half. the stock bov is super small, and doesnt vent fast enough for more than stock. so id go with the bosch upgrade one from 935motorsports, or if your lucky at pick a part down glenmore, search turbo cars for a bigger one. i found a wrecked 911 turbo and grabbed the bov off it, but you might find some turbo'd cars with n ice aftermarket parts you can mod to fit.
for boost controller what i used was a vacuum tee fitting. just find one that fits your wastegate hose, and cut hose in half and stick the tee in. and boom, 10-11 psi with a less than 50 cent part. and it cant fail!
if you can wait a month ill be in calgary end of november and i can give you hand modding your car