Trans doesn't go in gear. Now what?! Please Help

In reverse,throttle pressure is not used, nor even generated.So the KD mechanism might as well not be there.Line pressure will rise to what ever the pump is capable of. Perhaps 250 psi or more.So if you cant back up, with a little rpm, I would have to say( since the tranny worked before the TC swap) that the new TC is not working.
I guess I would get the rear wheels off the ground, and try reverse again, taking the rpm up to 2 or maybe 2.5 thousand. No drive means tranny out. If your shift kit is a manual shift, then manual-2nd(M2)should work, even without the KD hooked up.So while the rear is still elevated try it. No drive in M2 means tranny out.
I suppose it theoretically could be possible that the L/R strut fell out. So you might want to drop the pan and check this highly unlikely possibility out, before the tranny comes down.But without M2, I wouldnt bother.
If you dont have manual shift capability, then I would check the L/R strut, for sure. Unless you are confident that the band adjustment was fine, before the new TC went in
Its possible for the pump to be making plenty of pressure, and all the tranny guts be perfect,yet the tranny wont drive. The TC has to physically fit both the input shaft and the reaction shaft support, as well.
I dont think the RS supports were ever changed, but there have been at least 3 different input shafts over the years;the early, the late, and the lock-ups. So perhaps the TC was built or supplied wrong.