Offenhauser 4 barrel with what carb?

It depends entirely on which TYPE of carburetor you choose. You can run up to a 650, but it has to be the right kind of carburetor. I would look into one of those new little Street Demons. The 625 three barrel. They have very small primaries and a very large single secondary barrel. Since they are a vacuum secondary carburetor, they will only flow what the engine demands, so all you have to do is tune the primaries and adjust the secondary air door so that it does not open prematurely. I bet one of those would haul *** on a slant properly tuned.