I've had a couple of customers supply RMS front ends for their builds. One was a 67 Dart, I had to screw around with the engine mounts to get the engine to fit correctly. Was a pain in the ***..
customer probably supplied you with the wrong mounts.. i know when i did my alter-k the thing practically fell into place.. hell the car was a running driving car at the time and i kept everything engine, trans, exhaust, cooling system in place and it still easily fell into place. if someone has that much trouble installing an alter-k there can only be a few reason, wrong parts ordered, car bent, or maybe they shouldn't be working on cars to begin with.