Large RPM and Vacuum drop when shifting from park to gear

Hello everyone. I have a 1975 Dart with a 318 in it. The engine has headers, unknown cam, weiand intake, and a 670 holley street avenger carb. Transmission is auto. Have vacuum advance on my distributor, and it's hooked up to full manifold vacuum.

I tuned the carb in park with a vacuum gauge and got it to idle at around 1000 rpm and 18-ish inches of vacuum. When I drop it in gear, RPMS drop to around 500-600 and it dies almost immediately. To combat this, I tried tuning it in gear, and the greatest vacuum I get out of it in gear is around 6-7 inches @1000 RPM. The thing is, after I do this I end up with an Idle Speed at park of 1900 RPM!!!!

In park at 1000RPM and 18-ish inches of vacuum, the dial shakes rapidly but it is fairly constant. On the other hand, when it drops in gear the dial shakes rapidly between 5 and 8 inches. I could get a video of this and post it on this thread tomorrow.

My question is, any idea of what could cause this? What confuses me is the following: If I had an vacuum leak or another issue somewhere, would it only affect vacuum in gear or would it affect both?