FWIW, I don't think you should try to utilize the vacuum advance, they will not do anything for you, they need the extra vacuum to pull typically the last 12* of advance, the only time a vacuum advance will work is when the carb is too small.
My recomendation would be to leave it disconnected (no port cap on the distributor vacuum pot) and plug the port on the carb..
Or pick up a lock out bracket for the plate, either way time it for advance above 2500 RPM and don't worry about what you have at idle. I would say that you should have approx. 34-36* of total advance and all should be at least close....
As for the black plug, a compression check would be advised at this point. It may have been just a fuel fouled plug or bad wire

It almost sounds like you are having a valve issue what with the popping and whatnot, probably #4?
Good luck!
GT