Try Jawsgear, he also builds Supras.

Thinking of using an MKIII 8" diff, or the larger MKIV(8.6") diff in place of the 7.5" diff?

Given the weight of the Supra(especially the MKIII), 4.88's would work good for a road course track car or 1/8mile(might be just a bit tall), the taller gear would keep the rpms up, negating turbo lag.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed