Your gas cap should be made so that it vents. I've seen one or two that weren't, but those are very rare and they don't make your pickup behave well. I bought a locking cap because gas theft is a problem here and is vented really nicely. I checked it by putting my mouth up against the rubber part and sucking air through. I'll let somebody else advise you on how to modify that cap...I'd be inclined to look for one that was vented from the start.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end