There is a 2:1 ratio between the cam and the crank rpm. Every other rotation the marks will line up at 6 and 12 and the other time at 12 and 12. The 2:1 ratio is from the difference in the diameters of the timing gears. The block doesn't care where to install the marks (6/12 or 12/12), as long as they line up.
BBC's like timing. 17 at idle is good. 38 -40 total mechanical is good. The problem is with your vacuum advance (too much advance). Get an adjustment vacuum advance canister from summit racing and set it to add only 10 degrees.
You can use ported vacuum or manifold vacuum on the vacuum advance. Ported vacuum doesn't have vacuum at idle. Or at least it shouldn't have vacuum at idle, but sometimes does if the throttle blades are open past the transfer slots at idle (which is a bad thing).
Ported vacuum is an emission device so I prefer to use manifold vacuum. So, 17 at idle mechanical, then 10 more from the vacuum advance will give you 27 at idle. And around 50 when you rev it up in park.