I say when in doubt, go smaller on the carb. Especially when all you're running is a mild cam, stock rotating assembly and pretty much stock head. There's simply no more need for a larger carb on a six cylinder engine.
I opted for the 500 cfm Edelbrock only after hearing that it was easier to tune than the Holley and finding out that you can generally get them cheaper. Once I talked with Tom Langdon and got his "blessing" of approval with my setup them I went with it - Tom doesn't care for the 390 Holley anyway, for some reason. Still the Holley 390 would be a better fit "on paper."
-magic mike-
P.S. Check Ebay before you buy from Summit. I paid $250, shipping and all, about 3 years ago for my Edelbrock 1403 (which has the electric choke).
