Thanks for the info on the Getrag, guys. I've currently got it filled to the top PTO cover bolt with Amsoil syncromesh. I've only got a small open trailer right now, it doesn't really have any issue pulling that. Max weight that it sees right now is ~6k lbs.
Here it is pulling one of my friend's cars to a track day.
The camper shell came with the truck. The original owner bought it when the truck was new. It's an early ARE shell, possibly made for a Ford (judging on the shape of the rear door). I use the truck as my hotel room at the track.
The previous owner refreshed the truck mechanically and cleaned up the frame several years ago.
But here's a shot of the driver's side floorboard. Passenger side is the same story.
Thankfully, I found this D150 club cab shell with super clean floors and rockers! It's only got the typical surface rust on the roof and hood, plus a few dents.