Help for a newbie

[nwsteve]

First the good news. Found this 79 GMC 3/4 ton for $1250. Think I did alright. A real "grandpa's" truck, never even had a radio installed in it. All original from bumper to bumper.





This should be an easy one. I haven't done any crawling around uder this truck to chase this down (hoping to get an easy answer first) so can someone tell me how the dual tanks work on this truck. There is a switch on the lower dash which switches the guage between each tank, does it also switch the tanks or is there another switch for that somewhere else?
I got $60 of gas in the right hand tank I want to use!

Also have this module under the dash, never seen anything like before what is it.