Two other thoughts: the width of the driven gear in both the stock distributor and the HEI distributor is 5/8". Is yours for some reason different? I can't imagine it would be, but probably need to know. Also are you certain the bottom of the gear is hitting the internal flange? Can you see down into the oil pump connection? Is there any way something...anything...could have dropped into that slot? Maybe if you could use a bamboo skewer to measure from the bottom of that slot to the top of the block where the distributor flange will sit?
