To my eyes it looks as if the lower frame rails (including the kickup over the rear axle) are the original rails for the Pepsi delivery truck and the uppper rails were added some time later to let a later owner more easily install a more "normal" body. Th low rails let them put on a low body floor for the storage and delivery of the Pepsi product. This let them have a lower lift height, a serious benefit if you had to hand bomb all the product off the truck, even more so if you had to do it to load.
I don't know about that vintage, but some newer trucks of this type have a longer extension behind the axle to allow 2 (or more) bays ahead of the axle, one over it, and one behind. A quick google on "beverage body" came up with several sites with photos of completed trucks including
http://www.beveragebody.com/ but I didn't find photos of a dropped chassis waiting for body installation.
