Cheap Fuel Tank

[Tatey]

It may just be me misreading it but the auction says:

This unit is made to be mounted in upright position with filler neck at top so that the fuel sender reads correct as in picture

Does that mean it needs to be standing upright compared to lying down as in Andy's picture? If it does what issues would this cause with the sender? How hard would it be to put a new one in which would work which the tank lying down?