Either Holley is capable of flowing more CFM than that engine needs. My 401 V6 in my fire truck is also running a tiny 3-bolt 2 barrel, not sure if it's a Stromberg or a Zenith, and it runs OK with that carb. The 7448 is rated at 350 CFM, which will make your engine a lot more responsive at part throttle. The 500 will tend to over-carburete unless you learn to feather-foot it as the RPM comes up. Just a little too much throttle opening will kill the manifold vacuum and result in a bog, or even a backfire. The downside is that the 7448 is billed as a "race" carb in most catalogs and the prices are outrageous.
If you choose to continue tinkering with the 500, I'd suggest a power valve that opens at a very low vacuum setting, maybe 5 or 6 inches of manifold vacuum. That will keep the main jets in play longer without getting the massive rich dump when the PV opens up. Some of the power valves I have on hand are adjustable.
Jerry
