You most likely know that fuel tanks need ventilation in order to allow the fuel to continue to pump without causing a vacuum. This can either be through the gas cap itself, or via some sort of breather tube. The latter is usually routed up higher than the tank, before exiting below it, in order to prevent spillage if the tank is overfilled, or possible siphoning.
My lakes-racing Model A roadster pickup uses an Aeromotive 6.2-gallon aluminum fuel tank, with an internal 340-lph Stealth fuel pump. It has four ports: the ORB-8 outlet, an AN-8 return line port, and two AN-8 vents with rollover valves. I'd plumbed the latter two with oversize rubber tubing and hose clamps as a stop-gap measure when I first raced the car, but figured I'd do the job properly this time around, using AN-8 fittings and 5/8-inch braided hose to match the rest of the fuel system.
I also needed a pair of actual vents, rather than simply leaving the hoses open to the atmosphere. Eddie Motorsports came to the rescue with a cool pair of stainless steel marine vents. Designed for boats, they can accommodate a thick hull, and can also be bolted through sheetmetal, as in our case. They're designed for 5/8 rubber hose, so we welded some 1/2-inch OD stainless tubing to the ends, which would accept adapters to fit stainless braided hose.
Speaking of braided hose, this is a good opportunity to give a brief overview on how hose ends should be attached to braided hose, and show a simple tool that makes assembly a breeze. Just remember to use a little grease for lubrication, though try to keep it out of the fuel lines. These are vent lines, but we still blew them clear of debris using an air line after assembly. This step is even more important with actual fuel lines!