Why is my gas in my car, coming out of my vent?

I have a 1970 cutl s 350cui..3 years go I replace the gas tank ,beacuz it was leaking. i replace it with a brand new one it came with 2 vent holes, the original did not have no vent. so i cover both vents, there 's no were to hook it up. i take my car for a ride 20 minutes later my car dies i call a toltruck to pick it up and i notice under the car my gas tank was suck in so it crush my sending. so to make it work for little while i put a hose from the vent to the front the front or the car to charlcol canister. but it built so much pressure that come out of canister allot of gas. i cant figuired it out!!! please help