From the sound of it, you've got a good idea what to do. I'd skip the engine oil cooler unless you intend to do some fundamental engine mods, and you probably won't need it even then. I'd suggest, as others have, sticking with your stock axles; if you change the rear and not the front, you'll end up needing two spares.You'll also lose the crude but effective anti-lock brake feature on the truck. The stock 8.8, while not the darling of the offroad crowd, is a good axle that will serve you well. If you're set on changing the rear, swap in a 9" from the JY. It's plenty strong, and it's an easy swap- it bolts in with a different u-joint at the diff. The donor vehicle is an '80-'83 Bronco; The truck rear might even fit, but I don't know for sure.
As far as diff gears, 3.55- 3.73 is as high as you need to go. 4.11s will just make your rig scream on the highway, and unless you intend to do some rockcrawling, they're pointless unless you install larger (bigger than 31X10.50) tires.
Change the tranny and transfer case oil, and keep them, especially the tranny, full. the 5 speed doesn't take kindly to being underfilled.,
If you swap the radiator to a diesel unit as has been suggested, you'll need to swap th a radiator support from the same vehicle. I suggest you don't- the cooling system on your Bronc is more than adequate, again unless you're making funamental engine mods. The best cooling system mod you can make is to install a new radiator or have the original one cleaned at a radiator shop.
A set of quality gauges wouldn't hurt, either; I'd install water temp and oil pressure, and a voltmeter would be nice.
A little OT but I'll respond to the post about carbureted 300-I6's. They're neither doggy OR problematic unless they're ill maintained. I've driven my F150 for over two hundred thousand miles, most of it carrying a slide in camper; it's been a joy. Not as fast as the v-8's, but just fine.
Have fun! And I hope you find a slide-in camper for it........not that you were looking.:elkgrin: