Take a look at a couple of articles on this Corvette site (
http://lbfun.com/Corvette/Tech/vettetech.html) scroll down the page to "timing and vacuum advance." The article "Vacuum Advance Principles and Application" by Duke Williams and one by John Hinckley, "Advanced Curves" will explain vacuum advance, timing, and ported vs manifold vacuum. Big Blocks with big cams typically have lower vacuum than stock and usually need to have a different vacuum advance can installed. These two articles helped solve my problems.