According to William Wynne's Corvair conversion manual, the distributor has to be modified a little bit. The vacuum advance is removed completely, and the centrifugal advance is modified so that it adds more advance than stock. Also, the stock centrifugal springs are exchanged for lighter (weaker) ones so that the maximum advance is all in by 2500-2800 rpm or so.
Here's what I did to add more advance to the centrifugal advance mechanism. The shaft on the left is stock, and the shaft on the right has been modified by making the left-hand hole slightly more "egg-shaped".
Total Time: 1.0 hrs.