Some components are not necessary. The kick down switch doesn't care if it's 6 or 12 volts. It's in the low current part of the circuit and it will not make a difference - no need to change it.
The wiring is not a problem either as 6 volt wiring is 10 gauge on the high side and fused at 20 amps. As long as the 12 volt battery is not pushing more than 20 amps through it (and it likely won't) it does not need to be changed.
If you changed the relay, you may need the new wiring in order to hook it up and install but not because it's 12 volts.
