Posthole Digger old john deere post hole digger

[musselmark]

I bought a used john deere post hole digger and it came with a PTO shaft with a large spring wrapped around the end that goes onto the tractor. Also it has a shear bolt assembly with 4 holes, in one side the holes are almost 3/4" and the other side approx 3/8", what is the idea with this, the head of a 3/8" bolt will go through the larger hole, I attached a picture. I was thinking of drill 2 holes opposite each other for shear bolts. How do I determine the size, should I start with 1/4" or 5/16". The flanges of the shear assembly are pretty sloppy so that is why I think I need 2 bolts. The universal joint has no needle bearings and looks as though it would be impossible to ever remove, can they be changed