Depending on the power-draw of your radio, 6v-to-12v converters are available ($50-$150 or more).
Stovebolt.com has a good Tech Tip on 6v to 12v conversion. If you are going to want to use a lot of 12v accessories, the 12v conversion would make this easier. If you want to stay more "original equipment" and you will not want to use many 12v accessories, 6v works fine, as long as you have the correct size battery cables and ground straps (and clean connections).
12v requires lower-gauge wiring, so your 6v wiring will work fine in a 12v conversion.
