Won't start

If the puck is about 3" thick with just one vacuum line to it, then it is likely a vacuum canister for the air system, not a vacuum amplifier that was part of the later emissions. I don't think the '71' had the vacuum amplifier. Confirm if there is just one line to this puck thing.....

If you are measuring the manifold vacuum, then 6" HG is not normal at all. Confirm that you gauge is reading INCHES of mercury not something else. If it really is manifold vacuum, and really is 6" at idle, then you have a massive vacuum leak somewhere under the carb or in the intake or in the hoses.

The warmed up or not just has to do with the temp switched vacuum on the later engines.

The ported vacuum (blocked off in your case) is what would go to the vcan on the older engines. It ought to have 0" vacuum at idle. Other car mfrs in that era used manifold vacuum for the vcan; my 70' Ford did IIRC (vacuum retard at idle).

Oh yeah... I see you are in CA.... I get totally lost on the emissions mods for CA.