I've got a 360 with a whiplash cam, it idles at 8.5-9" at 700-750 rpm. Yes, it's a stick truck, however, while testing the vacuum assist on the power brakes, with just the engine idleing in the garage, I would repeatedly press and release the pedal as fast as I could, and never got it to run out of vacuum assist. I could feel it get a little firmer after 2 quick pumps, but it never actually got to the point where the pedal was rock hard like when stepping on the brakes with the engine off.

are you sure you don't have some other problem? isn't there a check valve on the booster? any chance it's backwards somehow or something? leaky booster?

try taking it for a drive and actually downshift the engine so that the RPMs jump up when slowing down. with the trans locked into 2nd gear at say, 45 mph, under a closed throttle, you should have MORE than enough RPM, and probably up around 30" of vacuum. If your brake pedal is STILL hard at that point, you need to look elsewhere than the cam's vacuum at idle.