I have 2 850s from blp, both annular boosters.
Blp sells a prepped 850-1060 cfm 4150. But if you're wanting to run a single carb I'd steer in the 4500 direction, I have one I will probably put some money into and test it on my 471 after I take a few pounds out of my intake manifold lol.
Also I believe dom sources his parts from blp
Lead69 has purple on his thumper carb, I was in love at 1st sight