Here's an old post:http://www.ytmag.com/cgi-bin/viewit.cgi?bd=ac&th=34442
If you have the original Chicago Rawhide seal, the new neoprene seal will look much thinner and may not last as many decades. Service manual calls for removing bearing cage in order to remove old seal and press in new one. Not to say you couldn't pry old one out with seal remover and slip new one over stub shaft of PTO. Either way remove any burrs from stub shaft. You will have to drain a lot of oil to get PTO empty. Should be 80W90; if it's dirty, replace it. Manual calls for use of sealer on outside of seal, which would mean good cleanup of seal seat. Bearing cage is separated from housing by layers of metal shims; these control end play of stub shaft and you want a free rolling fit, so don't damage them when you remove the bearing cage.
Not very tricky.