CFM is best figured as:

RPM divided by two, times cubic inches, times 0.85 (volumetric efficiency factor), divided by 1728 = cubic feet per minute.

Round it off to the nearest 50.

1728 = 12 x 12 x 12, to convert inches to feet.

If you over carburate an engine you get low end flat spots.

For a 351 it comes to 700 CFM if you plan to redline at 8,000 RPM.

A 302 (5L) is 600 CFM at the same redline.

Pick your normal operating RPM and plug in the numbers, you might be suprised - the above are full race numbers.

750 CFM Holleys were popular in the seventies with a lot of people who over-revved their engines and blew the crap out of them without ever achieving maximum efficiency.

Be a tech - not a schmuck!

If you could pull it off - I'd go EFI.

Standard sizes being what they are, I'd say go 750 for a 351, or 650 for a 302.

Believe it or not - for the MAX RPM you want, the numbers come out 500CFM for a 351, or 450CFM for a 302...

Those should be your ideal carbs