The amount you remove from the rear of the frame depends on what you want.
If you want to reinstall the bumperetts, or add a step bumper, that can mean a difference of a foot and a half or more. Also do you want to place the spare under the bed, or in a space left between the bed and cab.
You will need help (Man power, front end loader, or some other means to lift, move and place things such as the bed, axles, springs, not to mention plan on a lot of drilling holes, and cutting off rivets.
Measure, measure, then measure some more before cutting, or placing the springs. There is NO going back if you cut too much off the frame, or miss drilling one spring mount a 1/2" off.
Plan and think for as long as is needed before starting this, then plan on it taking at least a couple weeks.
You will most likely want shocks on the back, a 6x6 doesn't have them, but as a 4x4 the deuce needs them.
Take notes, make drawing, keep track of all the bolts, nuts, wires, and if you run into a problem, (Most likely you will.) like not having the correct tool, ask for help.
Impact wrenches, air supply, and specialty tools like this is your friend.
Ball Joint Press - Great Deals on Ball Joint Presses at Harbor Freight
Anyone ever write a manual on how to bob a deuce yet? Just wondering, for the most part, how much of the frame you have to cut out. I guess it is the rear axle you move forward, while removing the center one. I was just wondering how talented you have to be. Also, how difficult to remove the bed with minimum amount of tools. No bobcar, no backhoe, etc.
And how much does the bed weigh?
These are all questions that would be cool to find in one place. Has it been done, from start to finish, in writing? If so, I would love to read it.