On re-reading this,and with subsequent information given,I think there is a major clue here.
15/18 would be normal with a 262 cam, 22* lead, and 800 idle.
So IMO there is nothing wrong below the butterflies.Nor is the float level off , nor is it a pressure issue.
What happens if you reduce the throttle-opening by backing out the curb idle screw?
Where on the the throttle arm, do you have the return spring anchored? It should be on the same pin as the where the cable anchors. I drop it in between the arms of the clevis.
Check to see if there is sideways motion of the butterflies, that is to say;can you push/pull the throttle shaft from the drivers side, towards the passenger side?
Then push/pull the throttle-shaft away from the throttle cable.
I realize that the bushings would have to be extremely worn to create a 7inch drop in vacuum. I'm thinking more that the butterflies are pivoting in the bores, and pulling in air around the backside which is upsetting the T-Port sync, which with 22* of lead, is probably very fragile.
I suspect with the 22* of lead, the butterflies are very nearly closed, and any throttle-shaft play may upset the sync. This is why I suggested to reset the timing to 16* which IMO is plenty of idle-timing, for "a bit of a cam" . At 16* you will have to open the butterflies a bit, and reduce the mixture setting a tad. And this may put the butterflies in a less-sensitive position.
