Large RPM and Vacuum drop when shifting from park to gear

You likely need more timing. Start by adjusting timing with a vacuum gauge for highest reading, then see if the starter "kicks."

You need to post "if" the dist is recurved and what you have been running for timing, both total (no vacuum) and initial.

And it might be too lean.

And even then sometimes the throttles are open so far "with a big cam" that the blades are out of the idle transfer slots