Helping a buddy w/ 383 71 Dart, MM probs

I dont know about BB mounts but the SB mounts are keyed,so theres only one way to install them. And only one way they will lay on the K. And yes on SBs the rubber biscuits go vertical.Sometimes the brackets on the engine have to be pushed up tight to the block, before tightening the mounting bolts.
Have you tried one side at a time?