Reply to "650 Double pumper vs 750 vacuum secondary"

No.

A 770 carburetor is not too large. The most important consideration is calibration. You want to use a carburetor that is calibrated well for your motor's state of tune.

However, the four holes in the iron oem intake are small, sized for a 600 cfm carburetor, and will need machining to allow clearance for the larger butterflies of the larger carburetor. But then the material between the heat passage surrounding the holes for the primary butterflies and the primary holes themselves becomes very thin. I altered a few iron manifolds in years past, and to do it properly I brazed the heat passages, had the carb pad milled flat and then had the holes enlarged. A lot of work to invest in an iron intake manifold.

If the motor is reasonably stock, one route to increase the carburetor size on an M code motor is to acquire the intake manifold from a '71 or '72 Q code motor and a rebuilt 4300D Autolite carb.

If you wanna use a Holley carb installation of an aftermarket intake manifold may be easier than having an iron manifold modified. The Blue Thunder manifold is a good choice, but the Edelbrock Performer manifold will work OK too and they are relatively inexpensive to acquire on eBay. The new Edelbrock RPM Air Gap manifold is another choice in a dual plane intake manifold.

-G