One problem that can cause the distributor body to turn in use is if the gear is fitted too low on the shaft--- this causes the thrust collar to bear against the dizzy housing when its clamped down and trys to make the whole dizzy turn with it. Easy to check if you have a spare empty 351w block to trial fit the dizzy in to ensure that you have some vertical clearance in the shaft between lower face of gear & block when fitted . Not a bad idea to do this every time you fit a new dizzy or replace a gear. I suspect that this is the ''real'' reason for some roll pin failures that are blamed on
oil pumps etc.