I have run into something that I suspect might be common based on the way the hydro input shaft is made on the '77 300's.
In less that 50 hours of operation I have sheared 2 spring pins (1/4" x 1 1/2" acquired from Fastenal) that secure the drive shaft yoke to the input shaft on the hydro. On this vintage the shaft is just a plain round non-keyed shaft that takes the spring pin to make the connection. There would be a fair load on that pin and even with the fiber disks that act as the u-joint I suspect that there is some amount of additional torque and strain placed on that pin. It takes all the forces of turning the input shaft without aid of a key way or splines.
Has anyone else found this to be common on this vintage setup, or do I have something else going on here that I need to be looking at. It is not a tough thing to fix when it happens, but removing the fender pan each time is getting old. If this is going to be a common thing I might mod that fender pan and create an access hatch over that section.