The problem isn't with the transmission when converting to RWD. You could just install it with it's front drivepad not connected. The problem is fitting the transmission (due to the RWD body you have, the bulge for the transfer case doesn't fit) and the engine with it's transfer case. The best option is to get the engine and harness/ecu, then get the oil pan from an RB25DET and it's RWD transmission. That will fit in with only minor modifications.
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1990 Skyline GTE/S-t 4 Door Updated 6 Mar
Swapped out RB20E with RB20DET
Trans converted from Auto to 5-Speed...
Now with SR20DET 5-Speed
Installed Type M 5-lugs and brakes, R32 GTR rims and 2-Dr rear end (Tail lights & bumper)
1994 Isuzu AMU
US Spec Amigo with JDM Isuzu MU (Mysterious Utility, as it's called in Japan) nurf bars and hatchback style hardtop.