Use a hydraulic motor vs PTO - you can make a bracket and connect it with a loveJoy coupling to the shaft turning the digger chain. You'll save a lot on the PTO parts Gearboxes, sprockets and chain. Use a variable speed valve to control the speed and put in a check valvve to prevent you from turning it backwards by mistake.
Second clean up the points on the digger, rusty cruded up points won't go in the ground.
Go back to the tow behind setup.
If you must have a 3 point setup --- Remove all the wheels and use the smaller ones (byy some if you only have the large back ones) - install some brackest up next to the front point for depth control. The depth control will help a lot
Hopefully this will work. All but a few diggers are tow behind. They generally are much shorter.
Send some pictures to show us how you made out.