I think you want to look at this:
http://www.msdignition.com/pdf/85551...8547_dists.pdf
There's a way to adjust the max timing offset, by fiddling with the distributor, specifically the advance stop bushing. This should be the source of your problem.
There's a centrifugal advance curve that you can also set, by swapping out color-coded springs. What's yours at?