I would make that up to the deck you use and how much space you have between your bindings. I personally prefer packs to be low profile. If your deck flex a lot you need either a strong box like a pelican box or you might consider to make two 3s packs you connect via 10AWG silicon wire to give everything the option to flex if needed.
About balancing @malJohann can sure give a recommendation. He use lifepo4 as well on his build (think not the same cells thou)
You can have a look through his build thread as well