Read this, 307 motor mounts and frame brackets are different from the rest, I changed my '68 from 307 to 400 and had to deal with this also. Some headers need to be checked for clearances if you use 307 mounts & 307 frame brackets!

This only happens with a car that came with a 307.

---> http://www.chevelles.com/forums/show...frame+brackets

Pat