Hasn't this been covered before, I'm sure it has been covered before, anybody, everybody hasn't this been covered before?
Anyway, the easiest route is to find a donor truck or a good scrap/ pick and pull yard. Either way you go, get everything from that donor truck, every last nut and bolt, shafts etc. If you go with a mid 70's donor, the steering shaft from the column to the box will be the same, but get it anyways, may be in better shape, along with the idler shaft from the box to the axle. I found it easier to keep the lines plumbed in to the box, sometimes on the older vehicles they leak once you undo them and won't seal afterwards.
You can use a steering setup from an 80's to '93 steering setup, but there are a few differences, AS LONG AS YOU GET ALL THE PARTS, pump, brackets, box, steering shaft, idler shaft, etc it works. I'm currently running a setup off a '92 W350, I had to modify the steering shaft, as the rag joint on the '92 is at the steering column end, where the 70's are at the box end, you can buy a aftermarket shaft, or modify yours like I did. If you search for HMMWV tires on an M880, you'll find a thread where I did this, with pics.
Some tips,
Any steering setup off a Dodge truck or Ramcharger/Trailduster will work, as long as its 4 wheel drive, and it has a 318. Includes W100/150, W200/250, W300/350
Because you are replacing the box, you don't have to loosen up the steering column, but you have to insert the steering shaft before hand, as it dosent collapse enough to install later.
Refrain from spinning the wheel after it is disconnected from the box. Doing so will break the horn parts and it will work intermintally or not at all.
If you want to run HMMWV tires, you will have to adjust the steering stop on the drivers side of the axle, as the pitman arm fits nicely into the lugs of the tires and will cause damage. This is only on non-lifted trucks, haven't lifted mine, so cant speak to that setup.
If all else fails, search on here, this has been done several times and has been asked several times. Lots of info with pics
Good Luck