Nope- just take off the little cover that's on yours, and bolt it on. Probably want to use new O-rings when mounting. Be sure to get the hoses, outlets and mounting bracket for the outlets.
Nice find- those are hard to come by, and will add value to your tractor. Make sure the handle works smoothly, and returns to center. If not, just take it apart and clean up the spools, and it should be fine. Its very straightforward inside, and nothing springs out at you. Again, new O rings are probably in order. They get stiff with age.
Now just find a power steering setup and a swinging drawbar, and you've got a "fully outfitted" S55.