[VetteVet]

Wires off no power for ohms check. I think you can read the ohms between the end of the sender wire and a good ground if you don't turn the key on.

What is your sender ohms range. The stock chevy truck sender is 0-90 ohms. Zero is empty and 90 is full.
Is the Mustang sender by itself or do you have an in-tank fuel pump with it for fuel injection?
You can check the sender ohms by using the multi meter on the ohms scale and checking between the sending unit terminal and the sender mounting flange with it out of the tank you check it fully down and then gradually raise it up with the meter connected and read the ohms scale.