anything from 625 to 750-ish is going to be more than adequate. TQ's and Q-jets were 800+ and they came factory.
the avenger and slayer are not only different CFM, but the offer different details: the avenger has 4 corner idle, with metering blocks with jets on the primary and 2ndary and easy change 2ndary spring. the slayer has airbleeds and metering plate 2ndary. obviously there's a little more to it than that, but you get the gist of it.
i don't know your level of skill with tuning a carb or if you're even willing to swing wrenches, so my suggestion was based off of which two carbs i know would most likely work straight out of the box pretty well, be kind of close jetting wise, and be fairly easy to dial in with resources available online (here or u-tube vidyas).
there is also the consideration of what the planned use of the car is; a street/strip car, friday nite burger cruiser or touring car could ostensibly all use the same carb, but ideally each would have a carb best suited to their intended purpose.
and just to ruffle all the feathers: realistically, you could grab a used carter/edelbrock afb 625 for next to nothing, throw a rebuild kit at it and it would run just fine.
