Remember Super Shops? I got a set of motor mounts form them years ago to put a big block in my 71 Nova, and I had the same problem you describe. Turned out they were the wrong mounts. I knew that 68-72 Novas had the same front subframe as 67-69 Camaros, so I went to NAPA and got motor mounts for a 396 Camaro. They fit like a glove. Hope you figure it out.