340,
It is all about the tuning sequence. With a project liker yours, you do the idle timing FIRST, because all the other adjustments flow from this. Forget about the carb for now. As long as the engine will idle [ well enough ], do the ign test in post #53.
A 440 I set up, had similar idle vac to your engine, reqd 42-44* at idle. It was a turd with the factory 10-12* BTDC.
Also, you need to check the PCV if you are using one. It may be pulsating with the low vac, changing the amount of air been ingested at idle, causing havoc with idle adjustment.