76 gmc daul tank question

[71 Blazer]

My 76 GMC K1500 originally came with dual tanks, but the PO removed one. I bought this parts truck with dual tanks and am trying to restore my 76, anyone have a diagram for the lines and wires? Niether one of the trucks was hooked up right. What has me really confused is this "T" that goes from one tank to the other, only their is no fitting for it on the main tank(drivers side), when I traced it out on the parts truck it appeared to also be a vent for the front diff, dosen't really seem right, but I have seen weirder things. Also I need new tanks, anyone know where I can get a good deal on one? I need the larger of the 2 sizes, I think its a 20 gal.

 

[71 Blazer]

Been doing some reseach online, seems their are 2 types of selector valves. One has 3 lines, one has 6 lines. The 3 line one has only one wire, the 6 line one has many. How is it that I have a 6 line selector with one wire? Maybe I should just buy a new one?