"I can't get it to go in gear." I take that to mean that there is a mechanical connection between the crankshaft and pump, i.e., a PTO. I'm assuming a front mounted pump. If correct then that would seem to be where your problem lies. Lots of highway maintenance trucks used to be set up that way before constant drive hydraulic pumps came along. Goes like this:
Crankshaft adaptor flange to short driveshaft to PTO, then out of PTO to pump.
If a frame mounted pump then it still must have some mechanical drive system to operate and I'd look there if it were me.
