Just looked at Nige's PDF, i thought it was an excavator type coupling like on the bigger hystats and Tracked loaders. It's that poxy up-sized skid-steer coupling. I've seen them fail when they've been a part-spline out and some genius has still managed to yank the housing bolts up tight. Just trying to say that it isn't necessarily a seized pump.
Don't know whether this will transfer over as I haven't done one of this type on a dozer, but on SSL' and CTL's I find it easier to line things up by assembling the pump onto the mounting plate, fitting the nylon drive onto the pump, and then putting three or four studs in the flywheel housing so that everything has to go together square and in line. Never damaged a skid-steer coupling doing it this way, but seen a couple damaged following SIS. Just my experience, your mileage may vary.