Well... I've always read that you really don't need more than 650 CFM on any small block unless you plan on racing it. Heck, I have a 650 CFM on my 396 and it works just fine! (Of course, I rarely go past 3500 RPM, so the engine doesn't require anything more than that.)
That said, the original Rochester Q-Jet installed on the 327/275 and 350/295 was rated at 780 CFM, so I don't see why the Edelbrock won't work. However, the 600 you already have is just fine for the street.
One thing I've always believed in is never go too big with regards to carburetion. You'll get crap for mileage and the engine will be a pig at the low RPM's.