Assuming you have a one-wire selector valve below the cab:
Tie the new fuel gauge to the same wire that connected to the original fuel gauge; the tan wire (Ckt 30) that runs through the firewall and along the right frame rail then back up into the cab along the left frame rail, and on to the selector switch in the dash. Or, you can wire directly to the center cavity of the selector switch that is currently occupied by the 'tan' wire. On the single-wire selector valve systems, the dash switch selects the sending unit.
GM factory tank units are 0 - 90 Ohms, Empty-to-Full. Depending on the series of Autometer gauge you are using, you may need to use Autometer senders in both tanks.
Reference page A-4 (page 16),
AUXILIARY FUEL TANK diagram in the
1978 Wiring Manual.