This has been a problem that surfaces quite alot(well everytime someone replaces motor mounts anyway). My buddy just went through this on his 69. He noticed the issue when installing his new long branch mans. He thought the Mity Mounts might solve the problem. After installation and warming the car up in the driveway he noticed smoke coming from the right side. Looked down and the new MM was on fire. I think he ended up shimming his 1/8". My suggestion would be to try to get your hands on some NOS GM MMs. This should fix the problem. My guess is the replacements are just not as thick as the originals. Other than that shimming is probably your best option.