blazer tank install lines questions

[E.Furgal]

Hello.
I installed a blazer tank, the bed is off so I'm running lines..
the truck is a 71. the factory tank has two lines going from under cab to engine area.. didn't see a charcoal can, so my best guess is both go to the pump.. , trucks not here to look it over..

the tank has the filler neck and a small neck near it 3/8"thats the filler vent. I think?

the other side of the tank there is another 3/8" neck.. aiming toward the pass side frame..
what is that for..

I know what the one in the sender is.. lol

I'd like to add a charcoal can and a return line so if I ever go it efi it's there..

anyone have the factory routing of a rear tank lines,
I know I'll need to add a line for a return , and mostlikely change the sender line size to feed a 496..
any insight, thanks