That "disconnect the vacuum advance" note on that engine by GM is complete nonsense. Disconnect it to set the initial timing -- 16*BTDC worked great for me -- and then connect it back up to manifold vacuum, not "ported" vacuum.
http://www.corvetteactioncenter.com/...m-advance.html[/QUOTE]
Thanks Rich, I should also note this is a 4spd truck equipped with an sm465