You need to get the center section out of a 55-62 1/2 ton & replace yours in the original houseing. Set the pinion angle & weld the pivots by the springs so they won't turn. Get the drive shaft from the donor rear end & a front yoke to match the trans. Have a driveshaft shop put them together & shorten the shaft to the correct lenght. They really arn't too hard to do mainly because there is so much info out there. See the Tech tips & do a search. Ask any questions on this website. A lot of us have done them & the results are rewarding. Bowtie tim has done 2 on those models. The ones i've done have been the later model where I didn't have to change rear ends.
Great advice above. As he says you'll have to bite the bullet for the change to overdrive transmissions. First make sure your brakes are 100% before contemplating more speed, if you're already there that is half the battle, IMHO. I did exactly as Wrenchbender suggested by pulling out my center section and eliminating the torque tube for the open drive shaft and 3.90 conversion. As below in my signature 55k miles, daily driver only vehicle. Keep asking questions, a lot of us have been where you are.
