@Isaac nickerson,
I think you need to stop for a second, take a breath and then really think about, " What are you going to use your truck for and for how much?" Before you dive off into all kinds of mods that bring all kinds of associated changes and costs as well, it may be more trouble than it's worth if you're not going to really use all these mods. Some of those may be something you could start with like the rear brakes, but you may not need to change leaf springs and all of that if you really aren't going to need all of that.
There is the cost as well. You may not be able to find people who would sell you a bunch of things like rear ends, leaf spring sets for little to nothing. You may end up having to spend a bunch of money you may not have wanted to spend later on when you look back.
There is a lot you could do with what you have. 12 bolt rear ends can be modded with ring and pinion gear changes and a ton of other heavy duty stuff less expensive, and may work out just fine, "for what you really needed", and a TH350 can take a lot of abuse before you'd need to make a really big change like going to a 14 bolt. Look into modding the TH350, heavy duty clutch disc sets, torque converters, shift kits, etc., and maybe recurving
your leaf springs before going off the deep end with big everything just to end up driving down a dirt road and you never see deep sticky mud two feet deep, water up to the fenders, whenches, hooks, chains and all.
You know?? Compare what you've got first, beefing up the 12 bolt, your leaf springs, etc., before you chunk it all for something you may not need or need to do.
Don't forget all the suspension geometry changes you'll have to make when you start making lift changes, and the cost of doing all that when you go with bigger tires, lift kits and all, angle changes to the pinion as you lift your truck up. And what that will do to how it drives after all those changes. Do your homework, I mean really do your homework before you take anything off or make any changes.
But have fun and it will all be much more interesting and you will learn a lot!