In a GM distributor the mechanical advance pin is in elliptical hole in the weight plate and the amount of advance is limited by a rubber bushing. If the rubber busing has rotted and fell out, the distributor can be over-advancing. The pre-1967 distributors had a advance hole that is more elliptical than the hole in the weight plate of the 1967-up GM distributors. The 1967-up advance pin hole is nearly round which nearly totally limits the mechanical advance for low octane pump gas.
I am not confident this trick will help, but it can't hurt. You don't have to remove the distributor to do it. Use a hand mirror to see what is going on. If the advance pin bushing is missing, push a small piece of vacuum hose onto the advance pin in order to totally limit the mechanical advance.