Fuel Lines for 3 tanks

[Markedz]

Hi,
I have a 1970 Chevy Longhorn that has 1 tank behind the seat and 2 aux tanks (one on each side of the bed). When I bought the truck all of the lines were ripped out except the main line. There is a valve located on the driver's side floor board that could switch between the to aux tanks.
My question is this: Does anyone know how this was hooked up? It appears to be a dealer added option. Were there multiple filters? how do you switch between the aux tanks and the main tank? I have attached some pictures of what is mounted by the battery tray. Thanks for your help, I am trying to hook up all three tanks... These big blocks can suck some fuel