*****I know someone who did it. The mechanical part was not that hard as everything mostly bolted up. The hard part was engine management (fuel, spark, torque converter lockup, etc...). The car I saw had a wiring harness built by Caspers with some modifications. Driveshaft had to be changed to fit the 200R4 transmission as the original car had I think a Turbo 350 3 speed auto. Also the fuel tank and lines had to be changed to work with EFI as the old setup was for a mechanical pump. Also, the radiator shroud and other stuff in that area was modified or changed out to fit the stock location (CAS V2 I think..) intercooler, and the dual Spal fans (electric).
I remember the trans shifter was converted from column to on the floor using a console from a 1986 Cutlass/HURST/442. Oh, also the dash was from that car as it had really nice gauge holes sort of like the GNX, and it was easy enough to hook up the speedo cable, etc... I think he used little VDO gauges in the dash gauge holes for water, oil pressure, and boost. It looked GREAT. The car already had vacuum brakes but it was decided a larger vacuum reserve booster was needed due to the turbo, etc... so that came from an old police car which had massive brakes.
The front springs were changed as the motor swapped in was a lot lighter than what was in there before (I think a 305 V8).
I'm forgetting like 50 zillion things but the car ran great after everything was done and tuned. It had a MAF Translator setup as opposed to the more common aftermarket management systems out now which would simplify a lot and really open up tuning/mod options, etc..
I have pics somewhere of the car as I had just bought my GN when this was being finished up and thought it was a really cool project. It was a LOT of work and a bit of fabrication at the time that would be much easier to do now as we have so many more stand alone engine management options, E85, injector choices, turbos, etc...
Hopefully, you post some pics of your project.