Yes, it looks like a standard jd spool block for tipping trailers etc, you could just T your feed and return into the existing system and leave the original setup assuming that this suits your new setup.
If I remember correctly the pressure relief valve works at around 100 to 150 psi but that is very approximate. Most aftermarket loaders would not bother with 2 return lines, most would only return to the low pressure circuit.
Basically plumb feed from priority valve to closed centre spool...
I think your 3 pipes are
Flexible line is high pressure feed from priority valve
Pipe to filter is return pipe to low pressure lube and main hyd pump feed circuit
Pipe to top cover is low pressure return (only used when the pressure is too high in the low pressure circuit.
Also please remember...
There is a pin up at the mower end of the drawbar, move it over once the mower is in transport position. There are also safety locks on the lift rams, problem with both of these is the driver has to get out and manually put them in and out.
Edit, this is on a John deere 1360, 1365 etc mower, not...