I have done it and vowed to never try it again. I swapped the running gear from a rusted out 72 3/4 ton 4x4 to a 69 1 ton Longhorn 4x2. It took over 100 hours of labor. I had to change the engine cross member, front body/ spring mounts, drive line and axles, front shackles and mounts (the hardest part). It was easiest to do with all the parts from the donor. I had to replace the front brake lines at $100 for the pair. I also had to modify the rear driveshaft since the donor was a 8 foot bed and the new truck was 8.5 foot bed. It was a unique truck but for the time and money I said I'd never try that again. I sold the truck when I joined the Air Force but have since bought a 66 GMC Factory 4x4. 67-72 4x4s are out there so I'd just find one if that is what you want. Watch E-bay and local want ads for a cheap one to come up. Since you have the 78 4x4 remember that the wheel bases are different. 67-72 wheel base is 127in for a long bed and 131 in for a 73-87 long bed. If you decide to go with your plan of the conversion you are welcome to write me and ask questions.
