Do you even need a pump? You didn't provide any details, but if you are tapping into existing connections for a heater, I bet the cooling system circulation will work fine. The auxilary pumps in MBs are for slow speed running, only.
Try this, with no heater core, just route a length a heater hose as you plan, and see if it gets hot to the touch as you run your tractor. If it gets hot, then just cut the hose and install the heater core and you are done.
A devoted follower of the K.I.S.S philosophy.