Rob,
To get this project kicked off you need to know what size tire you want to run. If you are going to run a 21.50x33 then you need to design around that tire/ wheel combo. Here is a basic run down of the process I use.
1) Measure the width of your truck at the top of the fenderwells at the inside of the flange. This gives you the total width you have to work with.
1B) One thing I usually do is roll the rear flange up so I can move the tires a little farther outboard. I like the sidewall to be fairly flush with the outer sheetmetal and doing this allows you to move the tire out another 3/4's of an inch or so.
2) Measure the width of the tire, inflated, mounted on the rim width you plan to use. This is important and, unless you have access to a set already mounted, you need to have them on hand...
3) Subtract 6 inches from the total width for tire clearance (3 inches per side). I usually give the sidewall around 1 3/4's to 2" of clearance to the frame and 1" or so to the top of the wheel well after rolling the flange.
4) Subtract the tire width x 2 from the width left over after you do #3.
This will leave you with the new width for your rear frame.

5) You will have to have your tires and wheels before you can order the rear end housing. I actually never start a project like this without the tires and wheels.
You didn't mention a choice for your rear suspension but that will be a major factor and you will want to have all of those components on hand so you can design around them also.
A couple of things you can do is look at these sites. They have some informative tutorials and how-to's and will give you a good idea of what you're getting into.
http://www.cachassisworks.com/
http://www.artmorrison.com/

Mark