I’ll jump in here with opinions, right or wrong. One of the really good Chevy truck guys might run your numbers and put you on the correct answer.
If you can find and fit a 12 volt motor that would be the best option. A six volt motor with resistor is a band aid used when nothing 12 volt can be found and installed.
If no information is forthcoming, then bench testing would be a way to find out. You will need a 6/12 volt source. I have a heavy duty dc power supply for this. A 0-20 amp dc ampmeter and jumpers. I would load the motor by attaching the fan blade. Check you schematic to see what the fuse size is. Hook it up and see how much current is draws. Also a strobac would be helpful to measure the motor rpm. If the current and rpm are within reason at 12 volts, then run it.
