My requirements... what to build?

One thing I noticed, you recently said you plan to do a disc brake upgrade... if you decide to do that you will basically have no option but to upgrade the car to BBP unless you find a factory Kelsey-Hayes SBP setup which isn't that great tbh.

I put an 8 3/4" in my Duster but only because I found a complete one from a 1967 Coronet for cheap and early B-body rears fit perfectly in Dusters/Demons/Dart Sports. That rear end gave me BBP in the back and I upgraded the front brakes to the '73-up units which gave me BBP in the front. Every A-body SBP 8 3/4" I've seen listed for sale is usually around $1000 or more. Just for a stock 8 3/4"... really not worth it IMO unless you're restoring an original 340 car.