I am not up on the dana 80 rear swap, RonA can fill us in more there. But if you were to swap a 60F out of a second gen, that would be work. First off the diff is on the driver side on a second gen, this means you would need to change the transfer case. Second with the front, its coil setup on a second gen, and leafs on the front or your first. Lot of fab work there.
I can see why you are thinking about the 80 rear, but I think the 70 would be plenty strong unless you are going to go all out truck pulling with it. Even then the 80 can be broken. If you find a dually dana 70 out of a first gen, it has larger axel shafts, larger wheel studs and inner wheel bearings. (learned that from somebody here) The 60 front that is in the first gens is a good unit, complete with lockout hubs. It will be plenty strong, and can be upgrade to better shafts easily. Plus the wheelbearings are servicable, compared to the second gens that are not.
You need to advertise on the TDR classifieds that you want to change your 3/4 single wheel to dually. You could switch out the hubs for someones dually hubs to make the front for dually rims. I am sure unless someone proves me wrong that the dana 60f is the same for 3/4 and one tons except for the hubs. Then switch out rear ends, or buy a dually rear to make everything good.
You'll be ok, just think things through. Like said above, switching second gen stuff will be a BIG project. Lots of fabbing etc. No doubt would be cool in the end, but it might get you in trouble with the pastor.
Sounds like you need to figure out the transmission problem first, get it fixed, then move on to making it a real dually.
Take it easy friend, you are in good hands here. We'll take care of you.
Michael