Don't forget the factory limited the secondary opening on some of the applications for performance/emission reasons. Doing that does effectively change the CFM rating. My motor was a stock 350 2bbl and I put on a stock 68 4bbl manifold and a 69 Q-jet from a Buick Wildcat that had a 430cube and have the secondary linkage adjusted to 90 degrees at WOT. I think I have it dialed in as best as possible and the motor is handling it just fine. With the occasional running out of fuel at the top of first gear, but that's a separate issue.