Stock QJets are 750 cfm except for a few early 70s 850cfms off hi performance 455s.

Up to somewhere around 73-75, then they were all 850's.
Been told by "experts" that the 750's flowed 768 cfm.
Mayde, maybe not, but thier WAY better for econemy and performace.

Plug off that vacuum leak at its source and see if it helps. If it's from the A/C, factory source for that is a hollow stud on the right rear of the carb. If not in the factory location, trace the rubber vacuum line where it comes out of the firewall near the A/C box and windshield washer hose.