Well have you put the off set hangers on the frame yet? Stick them up there and see how much they interfere with the tank. You might be able to have the tank cut in half, and have the necessary amount taken out to clear the hangers. Or you might be able to just notch the tank in the corners where the hangers come over. Like cutting the corner of the tank out, inverting the cut out portion and weld the section back in. I hope that makes sense. It's hard for me to say what exactly needs to be done without seeing the car.
68 468 700R4, and here it is; mutert23