Welcome to the 'bolt,
6V starter will work fine in a 12V system, many successful examples including my own truck. A 12V starter will not properly fit a 6V engine (see
here and read the 9th post down from tclederman. He explains the tooth count difference between 6V and 12V applications.)
It does spin faster, be careful about cranking it too long if you're having trouble getting the truck started.
Have fun,
Bill
