There are several people working on these motors, but at this point using one is not easy as the electronics for the motor in the Envoy etc is fully integrated into the entire vehicle electrical system.
I BELIEVE the motor in the Chevy II wagon was built by Glen Self racing. Hot Rod and a couple of magazines had articles on the car.
No clue as to whether it would be quicker with a built 4.2 over a built late Supra motor, but it would definitely be an interesting piece. There was a long thread on modifying a 4.2 as a transplant here, so spending some time with the search function may yield the answers you are looking for.
