Columbia Specs

Hi,
I'm looking to buy a Columbia Glider raised roof. I will be pulling 53 foot van (50,000 to 80,000 gross with an average of 60,000). i want to get the best possible millage. I'd like the truck to get decent millage when going 65 -70. Obviously slower is better, but i need to get there sometime today lol. What ratio should I spec. They want to spec it with the dd500 hp (12.7l) engine and 3.58 rear. Would that combo give me good millage or should i get something else. Millage is more important to me then performance on hills. Also, would the ultrashift auto kill my mpg? I currently drive a 13 speed, but was considering the auto. Any other considerations to increase mpg?
Thanks,
Cody

 

Thanks freightlinerman