Assuming that the Carburetor isn't bleeding vacuum below the Butterflies when running, (Warped base or mounting surface) then if the Cam profile is the cause of low vacuum, you may need a vacuum accumulator to re-establish a 15-17 inch reading. This is common cause with Hi-Po cams if a external leak can't be discovered. Are the marks on the timing gears , accurate with the new cam specs? (Could need a thorough degree check to validate keyway relationships ,if any , to correct any flaws)
Porosity in any casting would have revealed itself with your other checks, unless the engine temp is an issue there.
Just some ideas , I'm sure others have more to add.
Pintosopher