I went with the 10 gauge. 40A should be fine.
I worked on the project again today. This time I worked on the pack. Doing it earlier would have been easy, but I didn't! So, I had to drill holes and make bus bars for the pack. This was interesting, but thankfully its aluminum and its soft and easy to drill. I held a small wood block under the bus bars while center punching and drilling which made it go alright. I also later marked the positives and negatives with a sharpie for easier identification.
