Mine is a 45 gallon aux. tank toolbox combo. Fuel lines exit on the DS of the tank, go through a hole I drilled in the side of the bed near the cab. This hole leads to the gap between the inner bed and the outside of the truck.
My tank has a sump and sending unit. I cut the fuel lines in front of the fuel tank cooler along the DS frame rail. There, I had to install a fuel switch and plumb the fuel lines such that I had both the OEM and aux tank plumbed into the switch and a single fuel path to the filter.
I also spliced into the electrical harness allowing the dash fuel gauge to read fuel level selected by a switch that I installed under the dash between my legs.
The tank is distributed by ATTB here in the DFW area. Works quite well. Installation instructions were very, very detailed and easy to follow. Cost for everything including some help during installation was right at $1k.