Bummer, I understand you are still using the mini HEI? Is it the same distributer you sent back? Perhaps the distributer is machined off center in a way that the distributer shaft is not perpendicular to the camshaft.Run a machinist rule down the machined part of the distributer in the same axis as the dist shaft,. Take a bunch of measurements around the dist with your verniers from different points.Make absolutely sure this distributer is not the fault. Your description of the half circle wear marks seems to imply an out of true dist? Then take of the dist cap and rotor,pull the plugs so the engine spins fast.Engage the starter and see if the dist shaft moves up and down, it does have some free paly right? Put some pressure on the center point of the shaft with a tool and see if you can feel the shaft walk up and down.
