The GM tbi's came in a few cfm's
they came on 2.8, 4.3, 5.0. 5.7 and 7.4
Pick the base gasket that fits your throtle body diameter
just turn it up side down and match the gasket to it
Most likely as you have a 305...it will be the smaller one of the two.
ωξψε ςμ χωψ