distributor shaft play

Be very sure that you are finding that play in the right place. Typically you can lift up and drop the dizzy cam a fair amount and that matters not a bit. The part to be concerned with, is the shaft itself, underneath of that cam.Also note that the play may be different between on the bench and installed. Again the installed play is what you are interested in.
I seem to remember tho, that on slantys, when the shaft climbs up the camgear, it goes into retard. Am I wrong?

Detonation avoidance is not limited to timing controls. It is often/usually caused by too much heat in the chamber. Lean running, excessive EGR (not an issue in 65 I know),and carbon buildup in the chambers, are all suspect. As well can be, to a lesser degree,a plugged exhaust, and excessive coolant or intake air temps.