600 cfm carb too small?

The 600 should work perfect for you, we have close to the same combo on an 81 notch that runs 12.70's all day long with a c4 and 3.55 gears. The 600 seems to be the best for all around driving with good response and economy.
The 650 we tried kept loading up and our et didn't change.

I run about the same setup in this new to me car...1985 convertible...and my older car from 20 years ago...1982 GT...and all good. I will say that in my '82 I did take the 600 vac secondaries off and tried a 650 double pumper and a 750 and it did run a little faster, but it also sucked alot of gas. I wound up bumping up the jet size, playing with the squirter size, and installing a weaker spring for the vac secondaries so they came on earlier. I am just now getting the '85 together up and running and went ahead and bumped up the jet size from the stock 62's to some 65's...smells like it is a tad rich at idle but I want to play around with the car before I make the decision on what to do.