I ran a few more iterations since the last posting. I'm really not sure how well wood ages strength wise, so I was looking for a little added assurance. I'd like the tank to last as long as everything else, so I'm looking at may years of service. Also, if I happen to get a small leak and the wood gets wet... There are many things that could compromise the tank, so a little extra beef isn't bad.
Anyway, I tweaked my pressure setting for a deeper tank which I forgot to do, and I also added some additional bracing. By adding a couple more 2x4s it is really looking good ($10 of wood maybe). The only high stress point is right under the bar that goes across the tank in the middle. Tomorrow I'll try running a simulation with correct loading on the sidewall and see how that effects things. It should reduce the stress on the cross bar. Until then, this is the current design using 15/32 plywood sides/bottom.