I think you just figured out how long "to big" is. At this length you may not be able to pull a long trailer. How are you going to turn a corner with this thing? Are you planning to put a different front axle for turning radius and weight? Actually this length may make your front end to heavy. I have a toter with a 12' box and it is very heavy on the front. I'd say that my wheebase is about 290" My box is 12' NOT counting the overhang. If you need space consider using a shorter box with slideouts. I'm not trying to poo poo your idea but that is a pretty long truck.
1989 FLD120 with 2000 NRC conversion