Lots of people trash talk the NP203 case but other than weight it's not a bad unit. However it does only fit a couple transmission which in your case is not a problem.

What is more of a problem with the 70's full time trucks is the front wheel bearings. Trouble prone and a pain to work on. Swapping to 80-93 knuckles is the preferred method BUT, they are a different lug pattern so new wheels and rear shafts go along with the swap.

You don't have to change things just keep this in mind.