By far the easiest is to use an early to mid eighties unit off a 4wd Toyota pickup. I think our host may offer a kit for this or if not do a search on this site and others like Stovebolt or H.A.M.B. This subject has been discussed quite a bit so something may come up if you googled "Toyota 4wd in AD pickup". The second easiest is to use the cylinder that attaches to the tie rod and has the control valve in the drag link. This was used on chevy cars up through 64, corvettes up to the 80's, and on Mustangs to 1970. Actually, the latter is probably easiest but does have hoses which is no problem on a non-lowered truck.
What he said! As one who took the other path (GM 800 series box mounted forward of the axle and outside the frame using a kit supplied by many of the old truck parts providers) I second Evan's motion to use the Toyota box.
Thanks for the advice.Gives me something to start with. I was wondering about the p/s from the Chevys with the cylinder because I have a 56 Belair with P/S and that has a cylinder. I'll try to find some of the other posts. Thanks a lot. Bob