I spent the entire afternoon underneath the truck. After another 4 1/2 hours working on it, I got the rear brake lines removed. The line running from the proportioning valve, under the engine and to the passenger's side frame rail was in great shape. Maybe due to being buried under 2 inches of oily crud for no telling how many years. After I got all of the muck off of the crossmember and off of the steel line, I reinstalled the one good brake line.
It went back in pretty smoothly and should give me a good point of reference to make sure I get the other lines installed properly. The rest of the rear lines are really rust and weak. I do not think they are worth reusing.
The only casualty, besides my knuckles - since the scarecrow is wearing my work gloves, was the rubber line that connects the main line to the smaller lines that service each rear brake. I was working around that hose, but eventually I realized that the line is 38 years old and not worth saving. Once I cut through the hose, I made much quicker progess, too.