Just a reminder, on the older front to back 2-4 intakes, they were designed for WCFB's or AFB's, so do some measuring if you use Holleys to make sure they'll fit due to their longer length over the Carters. Personally, I wouldn't have any hesitation (no pun intended) about using 500 AFB's. Adjust the linkage so that the primary throttle blades on the second carb tip in at 3/4 to 7/8 throttle. Unless you're turning some hellacious revs, the vacuum secondaries will just be along for the ride anyway. But that's just my opinion and it worked for me several times in my life. Your results may vary