Large RPM and Vacuum drop when shifting from park to gear

idle timing when it's running at 500-600 rpm? What is the ignition timing at 1000 as well?

If you are going by the book, throw those timing numbers out. Different cam and not knowing what it is is going to be an issue.

I would isolate the intake tract as well. Disconnect and plug EVERYTHING that is connected, PB, etc...

IMO, remove the advance from full manifold vacuum for now. Tune without it.