I'm probably saying things you already know, but a cheap belt from places like Harbor Freight or Tractor Supply is not going to stand up long under that load. You'll need a HP rated industrial belt.
The fellow that owns the shop I worked at is building (another) skid steer loader. It's powered with a 16HP engine that runs a tandem hydrostat that is powering four gerolor motors. Plus a 6 gpm pump and a 1.5 gpm pump mounted on the end. It has a dual pulley set up. He's using an automotive belt tensioner set up from a pick up truck. In your case you could have a lever to remove tension and releasing it would put constant pressure on the belts.
I also don't think the original pulley on the pto is capable of the load your machine is under, the pulley being too narrow, the diameter too small and possibly the clutch itself not up to the task. You need a wider belt Also six inch pulleys will be much better than four inch. Myself, I'd leave the original pto clutch pully out of it. If you want a clutch, then buy a HP rated centrifugal pulley. Expensive, but probably way cheaper in the long run.
Save the pump/motor set up you have now for the next project that you'll dream up just because you have them. I would advise dissembling the pump and motor and checking the housing and side plates for damage. Some housing wear is expected.
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