Depends on your location. Chert you can get here in west TN doesn't hold very well, it washes but it's cheap so people use it. Put crushed limestone on top of it and it holds fine.
PB from loader valve to 3rd function valve. 3rd function valve has a PB port that runs to rear remotes. Rear remotes power beyond feed 3pt valve then 3pt valve dumps to tank.
That is the sequence for just about any open center hydraulics in a tractor like yours.
Sounds like you might have some air in there. You drained the old coolant already and are flushing with clean water and radiator flush if I'm reading correctly. When you drained it did you drain out the radiator and the block? There is a block drain just forward of the oil fill cap.
When you say the block gets hot, how are you measuring the temp?
What does the temp gauge on the dash indicate?
It takes awhile to get a diesel that isn't under load to warm up and open up the thermostat. Infrared thermometer would be helpful for you to see what the temp actually is before you...
Sgt USMC 1991-1999. Probably would have stayed in but at that time they were drawing forces down so no reenlistment bonus in '99. Took my DD214 and GI Bill to college. Wouldn't trade those years for anything.
Don't grind anything, don't buy anything expensive. What you need is an over-running coupler / clutch.
1. Will help keep the momentum of the mower blades from "driving" the tractor when you throttle down.
2. Will allow you to make your connection.
Example over-running coupler
Positive side that fire in the woods will be good for the wildlife.
I'm still kicking myself a little for not buying one of those trucks at an auction one time. It was a Mississippi forestry commission truck pre emissions and low miles / hours. I flinched because it didn't have air conditioning...