As cobalt states you should actually take readings to determine the different aspects and limits of the distributor. Know what all the actual timing characteristics and limits are rather than just go by the way it runs, thats my take on it. I would think it would like at least 16-18 base timing.
And going along with what frisco is saying, some of the stock 390 distributors started to add mechanical at 400RPM! and were all in by 2000RPM with a maximum mech advance adding only 8 deg BTDC. See if you can find a part number on it. This site has some specs on the stock distributors
Sounds like a nice ride