Lil Debbie, the problem I found, was the washer caused part of the gasket to lift from the manifold making a direct vaccum leak. Ten inches is a little low for the street, especially in traffic.
As I put in my first reply, the brake booster vaccum line had vaccum flow, and the carburator effectively had a gap between the base, and gasket.
All I did was flip over the 1" gasket, torque to 40ft lbs, put a cable tie on the brake booster line, and it never worked better. The 1" gasket has steel bushings so there is very little crush.
I reccomend, removing the carb. You probably built it so you know where the vaccum hoses go, then just take a look see. Mine had a brown spot between the back venturi's, that is the EGR dump. The brown spot should have been dead center between the two back galleys, it was off center to the left rear.
She now runs like I designed it to. Power, fuel economy, and smoooooth. It is after all a 4 X 8 sheet of plywood upright with wheels.
I went from 3.23 rear to 2.73 posi. She cruises on the highway at 70 at 1600rpm.
check the base of the carb. Be careful retorqueing, it is basically a Q-Jet and over torque can crack the base.