Idle Rpm High in Park, Low in Drive
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.
That statement kind of explains things in a nutshell.
When reconnecting your vac advance to manifold vac, you are essentially advancing your base timing 20-22* (depending on your particular vac can) AT IDLE. Waay too much, which is why you're popping/backfiring out the carb- it's firing too early now.
Cap your vacuum ports. Set base timing with your vacuum gauge, as you have done. Adjust your idle mixture for best rpm/smoothest operation, then set your idle speed. Double check your idle timing.
Now reconnect your vacuum advance to ported vacuum. You should notice NO change to your timing. If you have an adjustable vacuum can, set it so the advance begins just off idle.
You will now have a smooth idle with no popping/pinging, smooth off idle operation with no stumble, and no appreciable RPM drop when put in gear (how much is dependent upon other variables, i.e. convertor, trans. fluid temp., etc.).
Now is when you can start slowly increasing your RPMs until you find your total advance, and at what RPM it occurs- tailoring this to your combo may involve changing springs in your mechanical advance, and/or the use of limiter plates, but there are hundreds of other threads on that subject- so at risk of turning this into a novel, I won't get into that aspect here.