Rick,
The fluctuating vacuum sounds like a burned intake valve, but a cracked piston could also be causing this problem on a 6 cylinder. Did you try the old "squirt
oil in the cylinder and recheck compression" trick? This sometimes will reveal a piston crack. Is there excessive blowby (crankcase pressure)?
Why not a 6 cylinder cobra? Just mentioning that would cause much heartburn to the Chevy vs. Ford FE vs. 351 stroker vs. 460 debate!
