A vented valved cap allows air in but not out. If you drill a hole in your cap, then fill your tank all the way up, when the gas expands it will dribble out and wash down the side of your quarter. If I were you, I would pull the filler tube and weld or sweat on a hose fitting on top just like the 70' non-emission cars had and run a line or hose down into the frame rail like the factory did. If you don't run a vented cap or vented line you also risk sucking your gas tank into a smaller shape.
I use a locking-vented cap under my flip-top that you can get at almost any auto parts store, along with my factory relief tube.
