i rolled my '90 gt, but i'd like to salvage the header, the head, and the fuel injector rail and put it on an 85 pony. is this possible and would it be worth the time it takes to put the gt head on the pony if it fits?

It will work, You'll need to transfer over the computer and harness also. You can swap just the head but that will lower the compresstion ratio.

will the lower compression lower horsepower? i dont necessarily need to put the gt head on there, but i figured i'd get more performance with the injector rail. is it worth puting the gt head on there? thanks.

Yes lowering the C/R will lower hp, but with boost you can make more.
Yes the GT head will flow better than the CFI head.
But the 85 CFI uses throttle body fuel injection, while the 90 GT uses MPFI, two completely different kind of Fuel injection system. The GT uses 4 while the CFI uses 1 injector, there is no "fuel rail" when there is only one injector.

"boost" as in turbo? sorry, im not too up on the mods.

Yes, boost as in Turbo.