A carb's CFM rating isn't "how much air it will flow", it is how much air it flows at a certain amount of manifold vacuum (1.5" Hg I believe). So if your engine "only needs 550cfm" and you put a 550 carb on it, you will be pulling 1.5" Hg, or about 0.74psi of vacuum. The more vacuum at WOT, the less dense air you get, and the less power you make. Now, this doesn't mean bigger is always better, but you gotta decide if you're willing to give up a little driveability for a little more top-end power. That being said, I just finished upgrading the stock 650dp on my 289 to a 750dp with the choke horn milled, and I've been driving it for about two weeks now and noticed no loss in street/part throttle performance, but it's MUCH stronger at about 1/2-2/3 throttle (just the primaries) and feels like it picked up WOT power as well, although I won't know how much 'till I get the jetting dialed in and take it to the track.
I think the 650dp when jetted right will make better power than your edelbrock 600, and even a 750dp would be fine, although you might notice a little loss in mileage. As others said, it also depends on if you're running an automatic or manual, heavy or light car, and tall or short rear end gears whether or not you'll really feel the difference.
'86 Bronco fullsize, 460, E4OD
'85 Ranger, 340hp 289, T5
<font size=-1>[ This Message was edited by: Motorhead on 3/28/06 1:33am ]</font>