Witch gas cap?

Which one fits tightly?
And likely more important ..............Have you done any mods to the tank evap system, IE the "carbon can?" There will be a 1/4" tube from the tank to the engine bay, and whatever else you do IT MUST be left open. Many guys plumb it into the air filter bonnet to help prevent smell. IT IS the vent to the tank in those years

Those caps are NOT vented. They are "pressure vacuum" and merely relieve excessive pressure in either direction.