Z32 transmission has got the be the best bang for the buck option. Its also probably the easiest to do/fit.
Key points to consider would be:
-Cheap
-Easy to find
-Have been proven to hold 800+hp
-Retains entire SR clutch/flywheel setup
-Retains factory SR starter
-Z32 transmission/alternator harness is easily adaptable
-Doesn't hang below the frame rails
-Adapter plate and shifter re-location are cheap
A little machine work, the right trans cross member and a new driveshaft get you a transmission that is much stronger than the factory one without any real downsides
