Do you have the vacuum line at the correct fitting. My recollection of an older SBC is that they used full manifold vacuum rather than ported vacuum like the Fords. The fitting should be below the throttle plates for full manifold vacuum. Static timing is with this line disconnected and the open hole plugged. Also, is your vacuum advance chamber good? If you can suck on the line and meet no resistance, i.e. doesn't move the point plate, it's "gone to heaven"