How much vacuum at idle? 750 CFM isn't too much. You probably found another problem in the carb itself.. Are these carbs Edelbrocks/carter or Holley type?
Edelbrocks need about 12 inches of vacuum to seat the needles at idle and also affects the cruise settings. Did you change the cam at the same time? If this is the problem, you can tune it out with a jet/needle/spring change and the time to work through it.
If Holleys, older power valves are susceptible to blown power valves.
Your combination doesn't seem to be excessively mismatched. If the carburetor isn't the problem you will have to check everything. It almost sounds like it could have jumped time one cog. Bring the engine to TDC and look to see that the timing mark is where it should be.Check all the vacuum hoses and manifold for leaks.
Originally Posted by dave1969chev
It will idle extremely rough and die unless I hold the throttle, no power at cruise, alright at wot but still not as much as I thought a head swap would give me. I tried throwing a 750cfm on it and it just blew rich smoke and wouldn't hold an idle. But I figured that was bc of the high alt. With my old heads I never had a issue with running a single plane, it behaved on the street.