Fuel tank issue 1967 c10

[Jeff1967c10]

I started rebuilding my little red truck I replaced the sending unit that was the only thing I needed for the fuel system at the beginning finished up the paint job and they put fuel in the truck the gas tank expanded so bad that I had to repaint the back of the cab because it cracked the paint replace the fuel pump ordered a new gas tank and put on a new vented lid the gas tank is like the original only one line going in It as a nonvented tank other than the cap now every time I put the fuel cap on the gas tank starts expanding I cannot leave the cap On while it is running or sitting with the truck turned off any ideals on what I can do to salvage the parts I have already bought