I changed mine (52 GMC, same seal) with the box mounted in the truck. You are supposed to pull the pitman shaft, but some of us have used a Lisle seal puller to drag out the old seal with the shaft in place.
Picture below is a leather stock seal being pulled with the Pitman shaft removed.
GM part number for the lip seal is 7801626. Federal Mogul 313842
It'll make your truck faster because the same seal is used on 1962-83 Corvettes.
I went one step further to make my 53 3100 go even faster by putting EPAS power steering on that is the same as the old Corvettes.
