Rear end swap

So I took the original 7 1/4 rear end of of my 67 Barracuda and am putting in a Moser fabricated 8 3/4 rear end. After centering the wheels the diff housing is only like 1 or 2 cm from the gas tank. I know the gas tank is not the original but as far as I can tell it looks to be stock size. Has anyone else run into this on a rear end swap? What should I do.... just get a fuel cell in the trunk? That's about all I can figure to do at this point.