Jeff, no Big ideas for you, but would not be a complete rear end of the the same donor truck be cheaper than a rebuild of your rear gears to try say a 3.7 or 3.9 ratio even if you have to cut/replace the spring mounts/.etc.?
Love my R-10 Mopar OD in my 50 4 dr.!
Too much changes for you, but my rebuilt 230 flattie w/ 3.9 rear gears runs great in town and freeways speeds, accelerates great for an old car.
No disc brakes but sure could use them!
Should not weight less than your truck?
4 sp. w/ OD with at least 2-5 syncho'ed -with the right ratio rear gears.. I think you would love!?? Run 4th. or 3rd. up hills!
Best,
DJ