Ford/Chevy mix-up

[dc59fleet]

When my original '59 gas tank sprung a leak, I quickly popped in a Ford version until I could fit something under the bed. However, the gas gauge does not like the Ford sending unit, or vice versa. The gauge registers in reverse, and only works for 1/2 the tank. When I reverse the wires, it doesn't work at all. Can someone that is electrically-competent please explain the problem so an idjut can comprehend? I'm concerned about what kind of sending unit I need to worry about when I put in the other tank under the bed. Will I have the same problems then? Thanks for the help.