The GenIII's use the Tremec T56 for this you would need at least a new bell housing, hydraulic clutch pedal and cylinder setup, new tailshaft and possibly modified crossmember, and speed sender adjustment device.
The aussie 5.7L strokers used the borg warner which are pretty much a straight swap (maybe crossmember and tailshaft changes though).
But if you can get your hands on the ZF 6 speed from the Group A's, that is the easiest one to use, if you're getting the tremec for a "really" good price you might as well snap it up as you could probably do a trade with a wreckers or something for a ZF if you wanted to, as the ZF's are expensive also.