Sorry if this doesn't belong here. My cousin has the FAST wideband setup and he now wants to put a gauge inside the car instead of having the box sitting out all the time. Will any A/F gauge work?

 

The gauge has to match the output signal from the FAST. Most wideband controllers put out a 0-5V signal, but many allow the user to specify the voltage output range vs. A/F ratio range, and many will emulate a narrow-band 0-1V output signal. Depends what the specific capabilities of the FAST are.