Way too many unknown to pinpoint the cause of the failure. I guess the first obvious question is if using a valve body that requires the throttle linkage, are you using the throttle linkage and is it adjusted correctly? Also, are you using a larger trans cooler, and are the lines clean and flowing good? Sorry I have to ask you since it sounds like the trans builder did not do the install.
If all that checks out, then you might want to question the trans builder? If he is cool, maybe he will let you watch him take it apart and and inspect the trans. That way you will know what went out, and maybe find out why?
I have seen a few "mechanics" that though they were transmission rebuilders, but I watched them just replace the frictions and steels, but not even disasemble the clutch pistons to replace the seals which were likely the cause of the failure in the first place
I don't know about the converter? I guess if it was pretty inefficent it could have generated alot of heat?