There is much more to a carb than just the peak CFM rating. I agree with Q-jet recommendation, and most critically that it MUST be a Q-jet specifically set up for your engine and power output. Q-jets were set up by GM to flow anywhere from maybe 550 to 750 cfm plus, depending on which engine they strapped it on...and easy mods open them up near 850.
Remember Q-jets were put on early 70's Super Duty 455's putting out near 500 hp, and they were also put on Mid 1980's 5.0L smog boxes choked down to a whopping 125 hp. And everything in-between. And they ran great on every application when properly tuned. That's the beauty of a Quadrajet...they can feed a proper fuel ratio to any CFM your engine needs. Other carbs can't do that.
If you're somewhat of a DIY'er, I highly recommend getting Cliff Ruggles book, How to Rebuild and Modify Rochester Quadrajet Carburetors, read it first, and then make a decision on how you want to go. Lots of decisions to make if choosing a Quadrajet...and here's a hint...the ones from the late 70's and early 80's on the smog choked machines are among the BEST to start with and rebuild to performance applications...they much better cores than the late 60's varieties that came stock on these cars. His book explains why, and how they need to be modified in order to be opened up to the full potential of YOUR engine, on TODAY's 10% ethanol unleaded fuels...while giving excellent starting/drivability/economy at the same time.
If you actually want him to build one for you, expect to spend big bucks, and get yourself on his 1 year waiting list ASAP...
Or read his book and do it yourself. He also answers his phone and gives tips to the DIY'er if you're smart enough to order your rebuild parts from him. He was an invaluable resource who I talked to a couple times during my recent Q-jet build for my 400, and I can't think of a better way to get it done. My Q-jet was actually a Carter model, pulled from a 1977 GMC Pickup truck. It was the perfect core, and it runs incredibly well after I did all of Cliff's recommended mods and put his rebuild parts into it.
