Building dozens if not a 100 or more street rods with ATL poly tanks and definately hundreds of Cobras, I to thought-fuel cell, your going to get an odor.....wrong! After doing all the appropriate things, new lines, fittings all fuel rated we still had the fumes. I then installed a simple valve that Ford used in the 80's-90's. It is rollover proof valve and simply pushes into a grommet (Ford) filled hole. Run an approprite fuel rated hose out the bottom of the floor not near any exhaust tubes and if you have already done the above this should take care of the problem.
Shown is a '40 Willys I built about 15 years ago and now bringing it todate: The roll over valve has been in now for about 8 years!
DV