Got Shell 93 Octane and now barely idles 1973 Dart 340

Solve the engine problem first
later the hissing booster connection.
Manifold vacuum can vary several inches with timing changes.and rpm plays a big role too

Here's the deal; with no vacuum leaks and vacuum numbers from 12 to 15 and an 850 idle, your cam is "not very big". This should be an easy tune.Prove, prove that there are no vacuum leaks.Prove externally, prove internally, prove the intake, the carb, lines, the PCV hose,everything. Blow out the low-speed circuit including the idle-bleeds.Then
I'll say it one more time;check and adjust your T-port sync. By doing this,the timing will settle itself/no more guessing.
I suggest to target an in-gear idle of 650, with neutral jumping up no more than about 100.

A bad shake at those vacuum numbers, would lead me to suspect a bad cylinder. Put the IR gun on the exhaust ports, and find a bad one, then check it's compression.There could be more than 1. Could just be a bad plug or wire. That would also explain your unstable idle vacuum, and why you need the high idle.
Unless the M-rods are not seated down at idle.
I think you're gonna need a timing tape, or a DB-timing lite.