The dealer should have a program to give very specific weight/balance specs based on specific engine/transmission/axles/etc. on the truck. A rough WAG would be rear wheels about 55% back on the box or about 45% of the box behind the axle centerline. That's a half asleep guess though and could be wrong but I don't think it's too far off. Again though, any dealer selling the truck should run the program that will give you dimensions, weights, balances, turning radius etc. etc.. Also, check what the price difference is between a 22' box and a 23'x" box. You can't add another pallet but you can use the extra 12-16 inches for load bars, blankets, straps and all the other stuff you'll need while still having the full 22' for freight.