Possible frame flex
 

I have a 66 Chevelle, Aluminum head BBC and when I raise it up in the air, I can measure from back of valve cover to the firewall and it moves about 1/8 of a inch. Solid motor and trans. mounts with 2 bars running from cage to front frame. What can be moving?????? and what do I do to fix it????? It does have stock front frame.

the only thing i can think of, is body mount bushing, if you now have rubber, you may want to try plastic.

sorry about that, ment solid alum.