Here's how I'm doing my 2wd 3rd Gen.Mine is a 2wd and I'm 4-linking the rear end so I didn't need to worry about the locations of the spring perches.
If you notice in the last picture the std long and short box the dimension A is the same. Dimension B is front to the rear axle. Most people that are keeping the rear leaf springs shorten the frame under the cab due to the rise in the frame from the cab section to the bed section. They just modify the rear cab support and weld it back to the frame.
This another build http://www.competitiondiesel.com/forums/showthread.php?t=121397
Here's how I'm doing my 2wd 3rd Gen.
Didn't cut under cab....
Not even finished but in hindsite wishing would have just 4 Linked it.
I have CalTracs, adjustable shocks, and rear sway bar. I also intend to run 12-14" wide tires in the rear.what are you guys going to do about traction!? on the street?