84 chevy

Hi guys, new here, just want to know if you can help me out. I am currently working on a 84 full size PU that had a 305, I am trying to drop in a 350 I custom built. I am having problems with the motor mounts. I am useing the original frame mounts, just replaced the rubber with prothane and put them back together and put on frame. I have the clam shells from the 305 and 350. I am trying to use the 350 clams as the 305 where filthy. I put them side by side and cannot see any difference. Is there a side difference, or are there a difference between the clams. When I drop engine down they will not line up or fit, seems one side will line up but the other will not. Also having a problem with trans, This could also be a problem I can get 1 side of tranny to come close but the other side will not.
I have put a tricked converter on, but it should fit a 350 I was told.
any help is appreciated