Jeremy on lime-swap.com can supply the modded harness and re-programmed PCM.

I'd suggest using a 4L60e as its the native transmission for the 4200 and the PCM with control it. Also its a direct bolt up. The TH700 would need an adapter plate.

The best approach is find a rolled donor vehicle so you can get the every thing including the small parts that you'll end up chasing.

All the boosted installs are home rolled..

51 GMC 4.2 turbo
Can't solved today's problems using the same technology/thinking that created them