A 12v frontcoil will have an internal resistance of 2.5 ohms or more. A 6v frontcoil will have an internal resistance of .3 to 1.5 ohms.
You always need the OEM ballast resistor on a frontmount; 6v or 12v coil doesn't matter. If you have a 6v frontcoil & a 12v conversion, you will need an additional resistor to limit coil current to less than 4 amps. You do not need any additional resistors w/ a 12v coil.
See tip # 30 at the link below.