14 Bolt GM's are usually cheap and plentiful as are truck Dana 70's. If you want to stay low, which I would with a six, and get some eight lug duallys in the process, look for a C-30 from '75 to '87, all the ones I have seen had 4.56 or 4.88 gears, and detroit lockers are fairly common, too. Of course, 8X6.5 inch bolt circles wheels are common as dirt in about any size you want. A retired step van bread hauler with an I beam or Dana tube front axle is your best possible donor vehicle, they are out there with disc brakes and you can get a matching system. Another vehicle to look at, an across the pond option so check parts availability and price if you find one, Mitsubishi/Fuso or Isuzu trucks. I don't know right off what the spring perch widths are but measure before looking. You may be get lucky (but don't expect to!) and find a direct bolt in. I would consider updating the springs at the same time as well.
