If its pretty much stock, then use the factory as baseline. They had figured out a thing or two about the timing curves.
I don't have a '66 book but do have a '67. I can tell you the mechanical advance for the 2 bbl looks noticibly different than the 4 bbl.
15* initial at 600 rpm would be about as much advance as would be wanted for a stock cam'd 383. So no, don't add manifold source for th evac advance on top of that. --