First thing to explore is the shifting mechanism and adjustment thereof. I believe that on your gearcase forward is engaged by the shift rod being DOWN. This works a rocker arm in the gearcase that pulls the dog clutch forward (towards the bow). If the shifting has been forced with parts not rotating, it is possible that something is bent and the dog clutch is not getting full travel forward and can jump.
If you have the gearcase I think you have, it is described in the Clymer "Chrysler Outboard Shop Manual" 1966-1984 as "Gearcase 20-50 hp." Many libraries have these manuals.