Idle Rpm High in Park, Low in Drive

Old cars are new to me so hopefully y’all can help me out...

Bought the car some time ago now and am working through lots of random issues.

69 Dart 360 Magnum swapped with a custom cam (no specs but appears pretty mild), edlebrock heads with 350 roller rockers (relashed), hei ignition conversion with stock 360 distributor (vac advance) and msd 6al box/wires, 650 double pumper (no choke), 727 with unknown converter stall (7/16 bolts in flexplate with half moon cutout). Motor is mechanically sound and recently rebuilt with limited miles. Trans has been “flushed” several times in searching for several leaks, fluid has always looked good. 91 in the car (highest we have here).

Crank does not have timing marks so timing is being done by ear/feel (I know I know) even though i do have a timing light. I have a vacuum gauge (15-16” at idle) and a wideband in the car (idle IN GEAR at about 13.5-14.0 afr, slightly richer in park). Intial timing, idle speed and idle afr set for highest vacuum, smoothest idle and best afr.

New issue im working through:

Car all warmed up and idling in park with vac advance capped, i set idle for 900-1000 rpm, put in gear she drops to 750-800. All is well.
Reconnect vac line in park, rpm spike to 1300-1600 rpm. Vac line connected and dropping into gear, rpm drops to almost 600 and car wants to stall and die, when it does die it sputters out the carb. Vac hooked up and idling, im hearing some engine noise I can only attribute to pinging (near #8 or 6) because it wasn’t there before and almost entirely goes away with vac disconnected.

Vac canister diaphragm confirmed good. It’s connected to full manifold vac (per everything im reading, including professional recommendations, full manifold is perferred to port). I have tried retarding/advancing timing with vac hooked but it either increases rpm slightly or makes the car die.

Warmed up, car is hard to start and pops some out of the carb unless I hold the pedal down and crank. Cold, engine will crank right off.

Is this huge disparity in park idle vs drive idle normal for vac advance? Am I missing something easy? Very new to distributors so I don’t know if the vac advance is simply too much for the car at idle or if I’m missing something...

I have read through numerous forum posts, articles etc and am still at a loss (now at a loss with a headache). I obviously don’t want to cap the vac and bandaid fix it but I’m getting close...

I plan to dump the oil and verify the noise i am hearing is not motor related. Will pull plugs and check for pre det/pre ign.

Any help would be great.