My experience has been that the main difficulty is aligning the splines which usually involves rotating either the
PTO drive on the tractor or the implement. Once I get them aligned the shaft will usually slip on partly and I can deal with seating the shaft fully to lock in place. Of course the safety shrouds on the tractor and implement shaft don't help with easy access either. I'd be very hesitant to use files, grinders, etc. to "modify" the drive splines - but an occasional wire brush cleaning and plenty of grease is good.