I know it's been a while, but here's an update.
The other 620 (the blue and grey one, named "Ol' Reliable") blew a head gasket after it got too hot (thermostat housing gasket was leaking). I sent the head out for resurfacing and had it checked for cracks, and what do you know, it was cracked at the number two cylinder. So that 620 is now being used as a parts truck for the orange 620. The fuel tank has been fixed and no longer leaks. After sitting for a while, the engine would not run (it did run before). After fixing a sticking valve and swapping the carburetor (it's a 32/36, basically a Weber knockoff) from the other truck ), it fired up and runs great.