To my knowledge, Sun Valley doesn't require a core, and whether they do is irrelevant. Either way, for the work involved, I recommend a new or rebuilt transmission, not merely an old one that fits.
Any monetary difference between those two units is lost as soon as you need to redo the work. And any '93 transmission that hasn't been rebuilt now needs rebuilding, 30 years later. So unless he puts a rebuilt one in now, he'll soon need to redo the work when it needs rebuilding.
You should probably sell yours to Sun Valley as a core and have them pay shipping.
maw