Holley secondaries won't open

Yes, a 600 cfm will be good for 300+ HP easily. I like vacuum secondary types because they drive better on the street and give better mileage in general, and works fine in my experience. I used to get 300+ HP from a 351C with a torque cam and 600 cfm Holley vacuum secondary carb, yet get 19-20 mpg in the highways. (Had some good aiflow mods too.) But, I never run engines to max possible HP either.

I was also thinking that a 500 cfm 2 bbl on a 2 bbl manifold that has been blended at the flange will do quite well, and should get you all that you can get from a stock cam and most mild cams too for 318 ci. This combo is common on circle track racing for limited HP V8 classes. The 500 CFM Holley is easy to tune. It might not be quite as driveable as the 600 cfm vacuum secondary though, as you don't have the small primaries which keep good airflow and operation at low loads and RPM's.