You are on the right track. If it were me, I would run a hose out of the power beyond port on your loader valve to the rear of the tractor. Then, you'll want a second hose coming back and going into where your PBY hose used to connect. You'll want male / female quick connects at the rear so when the stump grinder is not on the tractor,they will be coupled together so the hydraulic flow is not interrupted.... as this is what powers your three point hitch. Just keep in mind all of your tractors hydraulics are connected in series , so you cannot "tee" into the pressure side of hydraulic valves.
I'm assuming there is simply an in and an out on the stump grinder valve?? Or is there an in, out, and power beyond port ?
There are other options to operate the grinder, but that is the way I would do it. It may come in handy for you should you ever decide to add a backhoe or three point wood splitter in the future.