Saturday afternoon I drained the fuel from the F250 and dropped the tank. It didn't go perfectly smooth, but it's done. The front tank filled up while there was still about 5 gallons of gas in the rear tank. I remembered that I had a Boxster gas tank that didn't go to the dump yet, and drained the rest of the fuel into it. When the rear tank is back in the truck, I can use the fuel pump in the Boxster tank to pump the fuel back into the truck.
The tank itself was fairly straightforward to drop with the biggest problem being disconnecting the fuel lines on top of the tank. I had to pause and squeeze between the bed and tank with a fuel line disconnect tool and push the tool while pulling the hose - using both hands while propping the tank at an angle and assuming a yoga position under the truck. One hose came off as it should have. The other was stubborn enough that I cut the tubing at the fuel sender with a hack saw, then went back under with the tank removed to finish the job. The fuel sender unit I've had on hand is exactly the right replacement part.
While I was laying on the ground under the truck in 40 degree weather, covered in ~7 year old gasoline I tried to explain to myself why I do things like this. It's not my truck. I'm not being paid or asking for any favors in return. There's a lot of other stuff that I could be doing. It would be hard to convince anyone that it was fun. The best answer that I could come up with is that it's just what I do. My neighbor could have taken it to a shop to have it fixed, but it had already sat up to this summer since 2015 without being repaired, so not likely it would get done unless I did it. Somehow, I'm glad he let me. The truck itself was damaged while he was 'escaping' the 2015 Baltimore riots. The whole left side of the truck is scraped and dented, the driver's door was jammed and window broken (I replaced it in the summer). He has an emotional attachment to the truck for saving him and making it back to Oklahoma, but when he got back, it was parked and left unrepaired.
The fuel tank looked terrible inside, full of rust colored sludge. I don't want to buy a new tank or treat the whole inside with a sealant. There's a limit to my commitment and I really don't think he's going to use the rear tank much. Today, my solution was to swab out most of the sludge with the clothes that I was wearing yesterday cut up as rags (it's hard to get that fuel smell out anyway). With the sludge out, I used a power washer to blast the interior of the tank. It came out 'good enough for me'. I'm letting it air out tonight and I'll probably wipe it with some commercial alcohol to finish up before I install it. The inside isn't terrible now: