If by "looks like two half-circles running down the side" are you referring to a top and a bottom reinforcement rib on the side rail?(rubrail) If you are and it is a factory built body it could be a Knapheide brand. One of their models, std HD model, has that feature. It could have a wood, smooth steel or tradplate floor. It would have formed crossmembers. There are various bulkheads available to fit them. As for fenders to fit duals I would look to a semi-trailer dealer or a truck body distributor for drive axle fenders.
