No fuel

Diagram - http://userpages.chorus.net/elephant/FuelTankSelector.pdf - use this to check voltages where expected, test wires between switch and valve, etc.

My first guess would be that the valve is stuck between positions, and when you try to energize it either way (and the switch always energizes it with one polarity or the other whenever the key is on - see diagram), it draws too much current trying to move, and blows the fuse. If everything checks out electrically, I would remove the valve and blow in the various ports and check to see which ports are open to which other ports. Then I'd try triggering it with a test battery, switching polarity both ways. Be ready to pull one of the test wires quick if you wire it directly with no fuse.

That ^^^^^ indicates a break (infinite resistance) in the circuit between each tank sending unit and the gauge head. But once again, speculating here, if the selector is physically halfway between front and rear, then the contact that connects either the front or rear sending unit and the gauge head (see terminals 4,5 and 6 on the selector connector) could be in a position where it's not connecting either one. That would mean an open circuit, infinite resistance, and indeed a gauge pegged past full.