In some GM distributors, the limit pins are small enough diameter that a short section of tubing (brake line?) can fit over them and still allow free motion, limiting pin travel in both directions by 2 X the wall thickness.
I have this junk (likely repairable - but not by me) distributor laying around. So I’m pretty sure this is the pin and slot you are referring to. I can see how a bushing or piece of tubing slipped over the pin would limit travel in both directions. Probably wouldn’t need to be very thick.
So changing springs would change the rate of advance but wouldn’t change the amount of advance. Limiting the movement of the pin with a bushing will reduce the amount of mechanical advance.