If your tank has no fittings, you could also turn a furniture dolly upside down and place the tank on the dolly wheels with the tank against a wall. Use a fixture to mount a Sharpie and rotate the tank on the dolly wheels, it will give you a reference with respect to one end that is against the wall and the lines will be parallel. It does not have to be perfect, just close enough to weld up the gaps. The arcs you will marking will be less than 180 degrees.