Probably best to keep it the 700r4. Get it rebuilt with standard upgrades. Have the shop check the tv cable adjustment. If you change the trans you'll have driveshaft changes as well as a controller if you go for a 4l80e. It should be fine since your not racing or towing, besides with the lack of gas stations up near you Id keep the overdrive and add a second tank.
Transmission that end up in the bed of the truck are pulled for a reason, vast majority are just cores.
Is the listing in Pelkie a junk yard. looks like a lot of vehicles on the property, seen ads before but never stopped.