Post '86 chevy changed the heads so that the middle 4 mounting bolts go straight down instead of in at an angle - this is why your TPI unit will not just bolt on ( unless you change the heads too ). If you look at some aftermarket manifolds they are dual drilled for early or late setups, I have heard/seen people drilling the intake out so you can bolt it onto your existing heads. So theres an option.. or you colud find a '85 or '86 manifold base (which will bolt straight on ) and go from there.

Far as the electrics go you can get a harness from painless etc or there are a few guys selling them on ebay that make them themselves.

There are 3 main systems , '85, 86-89 and 90-92. The '85 is the most undesireable, the 86-89 is fine, but uses a MAF (air mass meter ) which you might not want under the hood ( can be kinda ugly ). The 90-92 is better in the looks department because you can get an air filter to just bolt straight onto the inlet.

What you could do is buy a 90-92 ECM, the sensors etc are *mostly* the same..

have a look at www.chevythunder.com - an invaluable resource ( the website owner is ne of the guys selling harnesses on ebay ).