Here is another thing you might want to check, that none of us have given any thought to.
Chevrolet Inline SIX-CYLINDER, by Leo Santucci, page 128. He notes.
"Distributor Pad Machining
Many 250 CID engines wore out the lower distributor bushing due to an inaccurately machined mounting pad for the distributor "collar". The pad was not machined perpendicular to the axis of the distributor bore, so when the hold down clamp was tightened, it twisted the distributor out of alignment. Always check this pad for squareness and remachine if necessary, or you will risk wearing out your "new" distributor. It is also possible to use a shim under one side of the distributor collar instead of machining. "
I know you don't have a 250, but this could explain a lot and would be easy to check with feeler gauges. You could even set up an indicator and see what happens as you tighten the dist clamp. You could even use two gauges. Side to side as well as fore and aft.
You haven't run it long enough to get the bushing wear he talks about, but it sure seems like it might cause the problems you have.
