Hey Jon,
I've attached a cheat sheet that I created when looking for stock carbs for my dual quad setup. Ignore the priorities; I'm not sure I'd rate them the same today. These are all Carter AFBs by model year and transmission. I'm running a 1964 pair specified for 425 automatic applications, but most any of the combinations should work. In fact, I've got a couple of mismatched Carter AFBs that I replaced with this pair if he's interested in buying a set to play around with. My understanding is all the Buick AFBs, at least for dual quads, are 625 CFM flow rate. So if he's interested in some other type of carb, that would be a good flow rate to look at. Of course, most anything from 500 CFM and up should work, but the factory spec was 625.
Good luck with your project!
Chuck