[rich weyand]

Buy a new 350/260hp crate engine, and put a dual-plane manifold and long-tube headers on it. Maybe put a better cam in it. Comp 12-235-2 is a good choice. Several others as well. 12-234-2. 12-300-4.

A 350cid engine at 5000 rpm sweeps 350*2500/(12*12*12) cfm, or 500 cfm. With a volumetric efficiency of 1.00, which a normally aspirated engine will not achieve, you could pump a maximum of 500 cfm through the engine at redline. Carburetors work best near their flow rate maximum, when stack velocity, swirl, and atomization are best.

So a 600 cfm carb is plenty.

That said, a Qjet with electric choke is the best carb for the 350. It's sort of the peak of carburetion technology, while still having a large following on the net that can help you tune it. There are some low-volume aftermarket carbs that some people like, but I would stick with the Qjet.