Saddle Tank Fuel Line Routing???

[SavageC20]

My 72 C20 has two aftermarket saddle tanks installed on it. I currently have them bypassed and am running on the stock tank in the cab. There is an old Stewart Warner electric fuel pump (model 240a I believe) under the cab. I am trying to figure out how these lines were routed and how it all works. Does the electric fuel pump fill the stock tank from the saddle tanks? What role does the three way valve play? It's my first time dealing with something like this.

Also, is this fuel pump worth rebuilding and running?