6 ohms should be fine.. 3 would probably even do it

my justification...

V=IR (voltage = current x resistance)

14V = (54W/14V) * R (54 watts came from 27 watt bulbs front and rear)

R= 3.63 ohms in the circuit stock..

of course i could be wrong.. anyone else whos done it already share what they used/ the results?