I ran a 750 CFM Holley on a 454, and it worked fine. It was a hot street motor - 6000 - 6500 RPM, 410 gears, 4-speed trans, full length headers.

Using the formula: RPM x CID x VE/3456, where

CFM = Carb size
RPM = engine rpm
CID = Cubic inch displacement
VE = engine volumetric efficiency (assumed to be .85 for hot street motor)

you get

CFM = 6500 x 454 x .85 /3456 = 725

I don't think a 770 would choke off your motor - but I'll bet you could make an 870 work.