1972 Gas tank vent problem?!?!?!?

[cjohnson6772]

Ok so I have a 1972 Cheyenne Super and ofcourse my truck came with the one year only tank with the emissions nonsense and the carbon canister stuff. installed a tuned port injection set up on my truck and ofcourse did away with the carbon canister. I removed the hard line loop from the passenger side of the cab and put a rubber cap over the tank fitting. I don't think my cap is vented because now I am getting a lot of pressure buildup and the tank is being sucked in until I relieve pressure by opening the cap. How cap I fix this without installing a new tank and without having a gas smell in the cab? Reinstall the hard line loop and leave it open underneath the cab? Can I get a vented cap? Is my '72 cap vented? Thanks for any help.