If you are running 80A, a 100A shunt is fine. If you run a bit more than 100A it will just heat a little and that's OK for short periods.
I like to keep a shunt and meter around for testing and setting things up. A meter made for panel mount is generally not so practical, generally needs voltage too, so I just use a shunt and my digital voltmeter. The photo attached is a 50A 50mv shunt: each mv read across the shunt is an amp. I just wire it into whatever circuit I want to measure.
I don't know why 75mv shunts seem prevalent. ?? Makes the reading an odd factor of amps, ie 75mv = 100A. So how many amps is 32mv.........??
Remember, a shunt is just a precision resistor. If you want to measure small currents just go buy a 1% resistor of the appropriate value from the electronic supply house and use the same way as in the photo.