Yes that rear will be worth a few hundred pounds in weight reduction as well removing the big wheels and drums. You may be underestimating the weight of the remaining frame, but until you start cutting and shortening that's hard to know.
Doing you homework first is the right way to start and while I have lots of opinions others may come along with different ideas.
I'm having trouble imagining a fenderless AD truck that hasn't been z'd and runs a dropped axle, but as you proceed I'm sure you'll show pictures of your project.
The disk kit you list is one of the common 1/2-ton choices. Its not one I've used, but I'm sure a little searching here will turn up others who used it and commented on their experience.
