My CJ gets 11-12 mpg driving 60 (Built 304, T5, 4.10's, 33" tires). Although they are different I bet the differences between tire size, gearing, engine mods will all shake out to a similar mileage.
You should have around a 15 gallon tank. I run out around 12 gallons in my '76 though. Not that I know.....
I'm running synthetic oil in my engine but I don't know if its worth it. I also run 20w50 oil.
Does your gas gauge work?
I would invest in a 5 gallon jerry can to take with ya. Also might check your points/coil before the trip.