Depending on yor flow requirements, an 850 should be fine with those heads. We have used 1050's on sbc.
th esmaller the carb, the more you choke the system, also, strangley enough the smaller the carb , the smaller the jets you need. Smaller venturi's make higher air speed past the bolsters pulling more fuel from the bowls.

i sent my 8896 dominater that was brand new ran three times on my 540 i sent it to patrick at pro systems he calibrated it to my engine specs and the car picked up almost 2tenths very good people there they even give you a number to call when your at the track and want to try and tune it even more they walk you through it. what carb shop will do that for you ?