yep.. you will have to do minor rewireing toinstall a volt meter.
wire the 2 wires together at the old ammeter.
wire a wire from the switched side of key switch to + of voltmeter.. and - of voltmeter to chassis.
key on = voltmeter reading.
me personaly? I prefer an ammeter.. it shows you the 'health' of your electrical system.. not some ominous volt measurement that MAY be meaningless...