To answer some earlier questions. I am running 16-17 base timing. That is, timing without the vacuum advance hooked up and with the idle slowed to about 600 rpm.
The GM HEI adds 20* mechanical advance, so I am running 36-37 degrees total timing.
The AR23 vacuum advance adds another 15* (spec is 7.5*, which is camshaft degrees), to make 51* all in.
51* is what the advance is at cruise, 2500+ rpm, with the throttle plate mostly closed (not accelerating), so the mechanical and vacuum advance are both in.
I have seen 52* given as an absolute maximum for cruise "all in" timing.
Check this:
http://67-72chevytrucks.com/vboard/s...d.php?t=689321