When you say functional what do you mean?? Aka, how hard do you intend to push the truck with that tire/axle combo.
Personally, if the truck is going to be used for hard wheeling and you want to run the 39's (might as well be with only .5" diff) you've got to think about what you want to do to it.
Stock 4.3 motor, or v8?? Atlas/t-case doubler, or low R&P (or combo of both).
The reason I ask is because in all honesty you're pushing a D44 with 37's - and that's with bling shafts/carrier/U-joints in it. While it could probably survive in the sand/sloppy muck with the 39's, if you've got any type of gear reduction/motor power being sent to it on dirt/rocks you're going to be breaking stuff a lot (including R&P's - I've seen it happen). Those are heavy tires, and once you add steel beadlocks they get even heavier. If you like throttle, this makes it even worse
Additionally, many people like to run full width axles on smaller rigs when they get into a tire of that size because of the stability this provides. If it were me, I'd run what Rocky suggested. If you're going to do the swap why go through the hassle twice? Do it once and be done with it (especially since tire size typically increases every time you purchase tires). Whether you want to narrow them is up to you - again, it really depends on your desired use for the vehicle. If you plan to run a large tire and keep the truck low, you'll probably be OK......if you want to run 16" lift and 39's I'd strongly consider full width.
As for the 9" axle, look into the 66-77 bronco's - they've typically got a 9" in the rear and a 44 in the front (well, the later years do). I don't know much about whether or not these 9" housings can be fit with the 35 spline shafts or not - that'll take some research. Also, many cars from the 70's were equipped with the 9"......check over at pirate and do some research.....the 9" is one hell of an axle - but again, you probably could do a 14 bolt in the rear for cheaper (it just weighs alot more and isn't narrow)