How about drilling and pinning the coupler onto the transmission and then slide it up onto the pilot shaft without a locking clip such that the transmission shaft holds the coupler bushing in place instead of the clip on the pilot shaft? With 3/4", it seems you could drill two 3/16" pin holes. If you have enough spline, you could make that work maybe without welding. If you don't trust the strength, then you might weld in addition to the pins. No matter what, I think you are going to have to take it easy with that tractor. I don't know if that coupler is going to take shock loads coming mainly through the PTO driveline. It'll be a judgement call and a bit of a crapshoot, but what the heck? You are trying to save a transmission. If it won't work, you haven't put yourself in any worse shape than you are now. Good luck. I don't envy you this decision at all.