Man if it were me I'd do maybe a camper shell with fuel tank like PA said going to the regular tank. I would stay FAR away from anything that might think its commercial. Don't need the DOT boys messing with you like they do with Semi trucks. So if it were me make sure to keep it just someone who is a DIY guy and not commerical money maker.
Oh and it might be nice to load it up with regular flashers something like what a civilian can have or like a tow truck. Never know if you might get in a traffic jam and need people to "think" you need through. Just don't cross over to the L.E. side which i think is colors only. We have some flashers on the white Raptor and it works good! Have Nosbusa hook you up i'm sure hes got ideas for that. Maybe some sound to go with it but I'm not telling you to do any of it....
With extra fuel get the 4.88s. Its not going to be much difference in MPG its going to suck anyways.
Get some rear facing lights.... Never know when they might come in handy if you know what i mean.
And while we are on lights and old trick i was told.... Put the brake lights if not all the lights on a switch. That way when you smash the brakes it doesn't look like you did and you might not get pulled over. All the rear lights you could put on a switch to if you need some help not being seen.
Oh gosh am i crossing the line I got more ideas maybe you should just give me a call...