I installed a 1968 Camaro gas tank in my truck. I thought it would be a good idea to put the filler under the plate holder. I took the truck out for the first time bottom driveway. Slight hill. Gave her some gas and laid some rubber LOL. I found the gas came out the back from the vent cap. Thought I would pass this on. I think I will have to change the filler to come streight up through the wooden floor.
Since i'm not using the original fuel pick up line I guss I can leave that open. Not sure about the safety
All tanks are vented through other means than the cap these days nothing unsafe about it. Here is info on venting a tank: http://www.tanksinc.com/index.cfm?ptype=results&Category_ID=160&home_id=-1&mode=cat
Put a hose on it and run it as high as you can ,maybe up a stake pocket,put a loop on it so the end is facing down to keep rain out of it .
If the tank is aftermarket the caps fit like crap and that issue is common I have found especially when using the aftermarket caps made somewhere west of California. I replaced my tank with a tanks Inc unit, they use their own proprietary cap that seals just fine, even under hard acceleration. Oh, they offer it in stainless too so you can polish it (the tank, not the cap).
...quit goin uphill and peeling out,..seriously, those tanks were used in Camaros and Novas and worked fine with the right cap.
I've taken a brass flared nipple, 1/4", from the hardware store and drilled a 1/4" hole just below the gas cap. Push the nipple theough and put 1/4" fuel hose with a new hose clamp. Run it about 3 ' and loop it through the frame a couple of times. Never smelled gas, never burned a car,,,,, because of that.