1974 MG Miget. I took measurements of my fuel tank to calculate its capacity. Of course with older cars there can be mixing and matching of different components (i.e in the '74 midget, I have from a previous owner a 1500 engine from a 75-80 midget and a OD from a spitfire). Is there anything inside the tank that can be displacing volume to know if my measurements of the outside of the tank are actually accurate?