The Chevrolet model number for that radio is 986067. A fuse around 10 amps should work fine. For running on 12 volts, you can use an inline resistor or a three terminal regulator. A resistor that is suitable is a 1 ohm 50 watt resistor such as this one:
1 ohm 50 watt resistor It is made to be attached to sheet metal to dissipate the heat.
One caution, if your radio has an electronic vibrator and it has been used on a GMC, it may be a positive ground unit. For negative ground operation the electronic vibrator needs to be a negative ground unit.
