[Rev]

I use a 625 Road Demon on my 306 and its just right. On a modified 351W, I think a 670 Avenger or even up to 750 would be OK for the 351W(depends on mods). I assume we're talking vacuum secondaries here. They are much more forgiving due to the fact that the secondaries only open on an "as needed" basis. I would never put a 500 CFM on a 351.

Rev