I had a Ford Econoline Van1972 I think (the last year they made the snub nose version) and I think it had a plain carburetor on it and it had a return line on it. The dealer did not know about the return line, I had a second fuel tank installed and they did not deal with the return line, usually not an issue as the 2nd tank was only used for short periods when the main tank was empty. But once topped off both tanks and decided to use the reserve tank, the full main tank was then pressurized and started spewing fuel out the filler cap at 65mph going down the freeway.
So just use caution and double check the fuel system design.