Vacuum reading help.

I agree with Redfish that most likely the vacuum pulsation is due to mis-fire. My 69 Slant was like that for years, and mechanics said "bad valves", "vaccum leak" and such. Turned out it was the carb and a ~4th one made it run perfect. That was a Holley 1920. Your Carter BBS looks easier to diagnose (have one on my 64 Valiant). If an ignition problem, the HEI may help. Also lookup posts about bad new distributor caps by SlantSixDan and others. The posts are ground off-center, which can cause strange mis-fires.