I have this kit, and always wondered if the guys at AMT forgot a floor pan for the cab.
The '57 truck is the same thing.
The model looks great, but they could had included a part to hide the interior tub from the underside, so it would look more realistic.
^^ Here is a shot of the under cab structure AMT/ERTL chose not to put in the box.
I agree with the comments (and have been intending to scratch up something to cover the sides of the interior tub when I build one of the 6 or so kits in my stash), but I think the OP's model looks great considering what AMT/ERTL provided as raw materials.