Update per my post above...
I have my Ford 22-63 PHD now working with my L3901. I went through a few designs/modification ideas but I was trying solve this without any permanent alterations to either the tractor of implement. Then the easiest solution occurred to me: just make an adapter to "extend" the toplink mounting holes -
The two plates are made from 1/4" plate mounted with three 1/2" bolts which allows some "fine tuning" up and down of where the mount hole are positioned. I had to modify the plates a couple of times so they would clear the u-joints on the drive shaft. I also originally had some tabs where I was going to weld on some kind of stabilizers but honestly the 1/4" plate seems plenty strong. If it starts to bend I'll look at making these out of thicker material.
To install, I run a couple of link pins in the outer holes so they are perfectly aligned and then tighten the three 1/2" bolts. Once the plates are firmly attached to the tractor I pull the pins back out and... voila, you're ready to mount the PHD.
The new link holes are 5" further away from the factory mount holes so this allows the PHD to clear the tractors arms but of course this screws with the range of motion of the PHD. Mounted in the bottom hole makes PHD too low so the auger drags on the ground even at full raised height. Mounted in the top hole the PHD can only dig to a depth of around 16" inches. Mounting in the middle hole seems to be the best of both worlds.
I've also noticed that when mounting in the bottom hole that the drive shaft tends to hit the frame of the PHD when fully raised.
Overall, not a bad solution for an inexpensive and well built digger.