I was up against this with my green Yanmar tiller. 1-3/8" 6-spline shaft on the tractor's PTO and a 30mm 16 spline shaft on the tiller. I had the short shaft that came with the tiller but it just wouldn't work with the standard PTO. The solution was to get a PTO shaft adapter on Ebay, 1-3/8" male 6-Spline end (with detent) x 1-1/8" female spline end. I think the 1-1/8" fits the old Ford 8n's? Anyway, a slight bore of the female end to slide on snug over the tiller shaft, weld it on to that shaft, and your in business with any 1-3/8" 6-spline PTO shaft. See photo attached.
Bill
