You can use brake line and there is 3/8"dia available. You could also use hard line and flare it with JIC 37 degree stuff and then use AN fittings. Dont use rubber hose for long runs, use it a flex points only. Keep the rubber part short if you can.

You have to have a switch in the line UNLESS the bottom second tank is above the bottom of the first tank and the top of the second tank is below the fill tube cap on the first tank....makes no sense, does it? Usually, you would plumb in a valve to switch the tanks, the fuel gauge wire, and the return line (if you have one). For a return line valve, an 85 or so Chevy truck has an electrical (solenoid operated) valve that includes the return line and the gauge wiring. This way, you can have a switch on your dash to change tanks.

There are also manual (lever type) switchovers that mount on the floor and go thru to underneath the truck. There are lots of sources for the manual valve. Look at LMC Truck for the stock Chevy switchovers.

mike in tucson