This is not a change I would recommend. Its a lot of work, and the truck is worth less when you get done than when you started.
You need a donor truck (73-80) with the full front 2WD suspension and steering linkage, and a replacement 2WD transmission and driveshaft. The rear suspension will be the same on 2WD and 4WD, so you just swap out whatever it takes to lower it.
I think I'd start with the 2WD conversion by pulling out the transfer case and 4WD trans, and swapping it to 2WD. Then go back and pull out the front axle and suspension, and move all the 2WD front suspension and steering linkage over from one truck to the other. You may have to drill some new mounting holes in the front frame, since I'm not sure 4WD frames have holes for the 2WD components.