Here is the wiring diagram for your (pre-1981) dual tank system:
Is it possible that the wire that runs down (on the driver's side) and back up to the cab (on the RH side) is tan - rather than pink?
It appears that this is the switched sensing leg for the dash gauge. But as you note, it looks like it needlessly runs out through the firewall, all the way down to the Pollak valve, back up the passenger's side and then re-enters the cab (without ever connecting to anything between the select switch and the gauge).
It would seem that this wire could have been led directly over to the dash gauge - without ever leaving the cab.