is there any ways to rig the mustang sending unit to the f100 gauge???

 

Yes, it can be done by matching the resistance from the sender to the gauge with an inline resistor but I am not an electronic expert so I can't tell you how to do it. Someone here has done this and should be along to help.

 

There are 2 basic styles of senders and gauges. The first is the Ford/Mopar style which is set to 73 ohms empty and 10 ohms full. The second is the GM style, which operates from 33-204 ohms.

I'm pretty sure the 56 F-100 gauge and the Mustang sender both use the 73-10 ohm circuitry so you should be OK.

Be sure you pay close attention to grounding the tank.