I'm going to a local shop tomorrow to see if they can take the clutch friction plate and make an adapter to a 6 spline out of it. I like the idea of using the splines on the gearbox shaft as opposed to drilling it and using a shear bolt.
Crazy timing for me to find this thread...
I just got a Howard Rotavator E4 series. Missing the tractor-side PTO shaft. I am thinking of removing the clutch and having an adapter made to fit the 10-spline shaft.
Did you have a shop machine the splines, or did you find a coupler to modify?