I agree with keeping and repairing the Quadrajet. Not that I don't think you can tune the Edelbrock to work well with your car but you already have the Quadrajet and they are excellent carburetors.

If you have the engine in the car and are going to get it tuned at a chassis dyno, get them to tune the car for the way you are going to drive it. Shooting for the highest horse power you can get isn't going to get you the best street/road machine. It's great to say 'I've got a 500 HP 400" but how often are you going to drive around at 7000 RPM unless you're racing? The beauty of a chassis dyno is the can simulate road conditions and tune your engine to perform for the road. Sometimes a dyno shop will keep changing the tune to get the highest horse power, which will come in at a high rpm, at the expense of low and midrange performance.

Sometimes people get hung up on that dyno HP print out which means diddly squat when your street manners suffer. Sometimes they get caught up in the CFM rating as well. Your engine is only going to take in as much air as it needs regardless of the CFM rating of the carburetor. One can tune a big CFM carburetor for a smaller engine but it's easier to tune a carb that is correctly sized for the CID and the rpm at which the engine is going to be run at.

And as Harold stated the HP rating Pontiac gave your engine was at the crank, you can expect the chassis number to be 20-25% below that. My 428 made 535 HP at the crank and it lost over 100 HP at the wheels.

Back to the CFM rating, mine made more top end HP with a 750 cfm carburetor than it did with an 800 cfm carb. and it's 464 cubic inches.