What drop did you do? 2/4, 3/5, 4/6, 4/7, 5/7, 5/8?
The spring hangers on a 2/4 mess with the pinion angle. If it wasn't corrected via pinion wedges, that'll eat up U-joints right quick.
My trucks, one with a 2/4 the other 4/6, then made 4/7, never had probs with the driveshaft bottoming out in the trans.
That WAS a concern on the 4/7, so we checked, double checked, then checked again. Said screw it. Put it together, & drove the snot outa it for 2 years. No probs!
One thing no-one has mentioned that has crossed my mind.
If the 'mechanic' didn't tighten the bolts on the TC properly, did he get the bolts holding the trans to engine tight? ALL of them? Are the line-up pins between the 2 seated?
Is the torque convertor even seated on the keyway to the trans?
All or any of that would push the transmission back enough to create the prob with the driveshaft. The driveshaft does have to have some movement back & forth, but on a lowered 2 whl.dr., it isn't all that much. It will sit farther into the tailshaft, sure, but there should still be enough room for it to slide back & forth. It'll just be in a different position farther up the yoke. Even still, there shouldn't be a problem with it bottoming out inside the tailshaft.
Did he have a tough time getting the transmission mount to line up with the trans too? That'd be one indication things aren't quite right.