Replacement fuel caps for '71 Chieftain

[Denhamt1983]

While driving our '71 D24 Chieftain cross country from Colorado to Georgia recently the seals on both gas caps (older, non original locking replacement caps) went bad and I had to acquire replacements on the road with "it fits!!" as the only requirement for fitment. Turns out the new caps are only vented in one direction (for vacuum) and the tanks are unvented, so now I have a ton of pressure building up in there leading to violently popping tanks and/or fuel forcing it's way into the carburetor and leaking while the Winnie sits up. Does anyone know the part number for the correct fuel cap, or at least one that fits and also allows venting of pressure to atmosphere? Much thanks!