Les, my 51 3600 has the original 4.57 geared HO72. When I replaced the leaking pinion seal I removed the 1 5/8th pinion nut (thin wall socket required, I made one with the bench grinder, a cotter key also has to come out) then removed the pinion bearing housing bolts, then tapped it with a hammer. No drama, just work.
Also don't forget to mark the pinion nut before turning it so you can tighten it to the same spot on reinstall.
Good Luck
