Brent
I have one built from a 40 hp 5' king kutter. I believe there are a few threads around here on it. I had to reinforce the deck quite a bit as the down pressure of the loader was bending the front of the deck up like a ski. The crazy wheel also required quite a bit of improving
Forget the over run clutch, your working with hydraulics now, not that ancient parts smashing shaft drive. The problem is simple to solve with a check valve, costs about $15. When shutting the cutter off it lets the pressure in the return hose loop back to the input side of the hyd motor. Under normal operation the check is forced closed by the pressure from the loader so the oil must go through the motor instead of the loop to the return hose.
The actual skidsteer cutters are built with a lot more steel in the deck. Generally 1/4" instead of 1/8. A front roller made of steel and most have a floating linkage to fasten to the loader.
I can't say I recommend the tractor cutter route for commercial use. But I get by with it.
The gear box has been fine as hyd drive is much more forgiving on gears and shafts.
Of course I'd love to sell you a new skidsteer one, but if you want to build you own I can give you a few pointers there too.
Ken
I see TriHonu beat me to the punch