Venting the cap would work just fine on many vehicles.

But on my 32 high boy the cap "is about level" with the fuel level in the tank when it's full so the fuel is on the verge of running out of the tank if you park on an incline. Not the best seniero to vent the cap.

So what I did was vent the tank to a 5/16" steel line that I bent and routed up through the trunk floor, straight up to the top of the trunk (inside), bent it back 180 degrees and routed back down through the trunk floor and then routed it forward to the inside of the frame rail. Then I drilled the frame and taped it for 1/8" NPT. I used a 1/8" NPT FORD inline carb fuel filter screwed into the frame and attached the hose end to the 5/16" steel line with a short piece of fuel line and two clamps.

Incoming air, is vented/filtered by the fuel filter, then the height of the steel tubing keeps the fuel from sloshing out. No fuel smell is inside the car because it's vented to the outside, even though it's plumbed inside. It does not have a one way flow valve, but when and if I need that I will have BIGGER problems to deal with! Tested for 8800 miles so far.