Easy way to do it on an old Mopar Distributor is to wedge a piece of ~.1-.125 header gasket material between the pin and the slot, takes out about 10-12 degrees of centrifugal and if done properly it can't go anywhere because the pins sling against it as the distributor speeds up. You could weld up the slot but back in the day this was easier. This allowed to advance the distributor and keep the total timing ~34-36 degrees.

It's real good and easy to try to see if it makes the difference you need, you could always weld the slot up later if you want.