We went through this same deal with the 72. I already had a RMVB so we put it in. Dad was missing gears all the time.

I started looking for ways to modify the slap stik to work like a true ratchet shifter and never did come up with an acceptable solution.

In the end we installed a TA auto/manual VB. It's like it was made for a slap stik. DIdn't see any difference in shifting or performance between the 2 other than now it's nearly impossible to mis a shift.