I would get the highest, widest, and longest box you can get, that will fit on your truck legally.If it is a used one, make sure the floor is made to withstand a towmotor.Some are not.Check for fllor wood thickness,crossmember spacing, etc.Also, get thebiggest door opening as possible.I had the back end of my box cut off and a semi trailer frame and doors out on, so the door opening was the same width as the inside box,and just a little shorter on the height.Phil Robertson in Mt.Vernon, Oh has good used boxes all the time, and will modify them to your needs.Real good people.(Robertson truck sales) on rt 36, east of the Flying J in Sunbury.