fuel tank problem

The fuel tank in my 71 elky will not fill up more than half way. the tank has a hose fitting on the upper front corners and a fitting in the middle of the tank. all of these hoses go to a plastic box above the fuel tank. the evap canister is long gone and the hoses are plugged. I was thinking of running a hose from the top left hose fitting to the gas cap area and installing a differential vent on the end of the hose. any thoughts or different ideas?? thanks!!