I can't help you on the manifold vacuum as I have never used power assisted brakes on any of my half way fast cars or customer motors that needed decent vacuum
My old 518 C.I. pump gas six pack motor had less than 6.0 inches of vacuum at or below 1000 RPM with the solid roller cam that was 260@.050 on the intake and 266@.050 on the exhaust lobes ground on a 108 LSA installed at 106 intake lobe centers The only reason I know that is trying to get the six pack to idle real low (800 to 850 RPM) in gear to show some of the local "experts" that you could make a Holley carb. idle like that with a little effort
Have you thought of using a electric vacuum pump to help on your brakes