X2 on the Q. :yes
Everyday has a good rep. According the site, he has an 800cfm '76 Pontiac Q ready to go.
http://www.everyday-performance.com/quadrajet_carburetors.htm#!/Pontiac-1976-Trans-Am-Firebird-400-Quadrajet-17056264/p/46561051/category=578011
These '76-'79 are desirable because they have the APT feature and they are 800cfm. Since you are not running a TH350 Trans, you will not need a Q with the throttle lever which has a provision for the TH350 kickdown cable.
So, that '76 should work for you. BUT, the '71-'79 Pontiac Q's had the big "smokestack" vent, which requires a hole for it, in your air cleaner base. If you don't mind drilling a hole in your base, for the vent, that won't be a big problem. Probably also is a good idea to use a rubber grommet to help seal the vent hole.
If you don't wanna fool with the vent, you can go with a '68 thru early '70 Pontiac Q. OR, you can go with a Buick Q. Everyday has several of these ready to go, in both 750 and 800cfm versions. But, you may have to modifiy your throttle cable hook up, slightly. And, as you can see in the pic linked, the return spring on these Buick Q's pulls from the front, not the rear, like Pontiacs. Some guys are OK with that and some are not.
The 403 Olds Q's also have the APT feature, but without the big vent. And there were lots of 403 Olds engines, in the '79 Birds. So, these 403 Q cores should be easy to find and fairly cheap.
Another consideration is the choke. If you will be needing a choke, I'd consider using a Q which came with a "hot air" type choke, mounted on the side of the main body. With these type, it is easy to replace it with an electric choke. I think Pontiac began using the hot air chokes in '73, maybe. The electric choke can be added by the rebuilder, or you can buy 'em and add it yourself.
Another good Q source is SMI. :smile22:
http://smicarburetor.com/products/sfID1/28/sfID2/9/sfID3/100