A Ford 4500 industrial has 23gpm on the loader circuit. That might be enough to run a mower. I'd do the calculations first (or look up the mower manufacturer's recommendations) before going down this road too far.
The PB line should be a line from the loader valve to the rear 3pt. Often the 3pt cylinders are in the transmission case, at the very back.
You may want to add a rear remote valve that has a detent and a motor spool. The mower manufacturer should say what sort of valve to use.