A built C4 w/ manual valve body would bolt up to the V6.
The computer is ok, it doesn't control the trans in an early 89-93 SC.
The TV cable controls the tranny when it shifts, etc... with a manual valve body the TV cable isn't used. YOU control when it shifts, and have to shift it back down when you come to a stop. The computer is only going to control the ignition timing and injector pulse etc...
I'd probably want to stay with an AOD, you can get a strip terminator from lentech w/ brake and manual valve body and then you'd still have OD if you ever drive the car on the street. With an AOD I'd say 4.10:1 gears.
I am wondering what 4.30:1 would be like in an AOD but I think with a 26-28" tire you might have to shift to OD in the 1/4 before the traps and that's pointless.