if you are going hydrualic. *rubs chin* wondering why you are wanting to keep the gear box.
properly sizing the motor and pump. can result in what you need. without ever needing a gear box.
check out some stuff at
Surplus Center there is a lot of choices there vs other websites. might help ya figure it out. in the help section some place is some online calculators. to help ya figure out cubic inch info to figure out RPM's.
the more RPMs = less torque
the more torque = less RPMs.
540 pto pump X pressure (what ever you want) at Y cubic inches = GPM it will create at max pressure.
on the rotary cutter motor side....
blade tip speed / rpms = wanted motor RPM.
RPM and GPM get those numbers and figure out what cubic inches the the motor will need to be at.
if you were going to run it off say a 3rd function connection on FEL. you get GPM and max pressure from your tractor specs and not a PTO pump on the rear.
inside diameter of pipes and hoses matter. going to small = lots of extra friction and loss of pressure to the motor. going to large and you just waste cash. NOTE inside diameter, not outside diameter of hoses and pipes.
your pressure relief valves, can help reduce sudden shock of hitting something.
--an accumulator. can also act like a shock reducer.
--hoses themselves *meh* longer the rubber hoses. they can expand a little bit to take shock, but would not rely on it. more likely to bust a hose doing that.
--water hammering effect = google search. most of the stuff will be for homes. but might be initial learning to understanding hydraulics better.