I have a few '66 Chevys and one thing I noticed when looking for bed parts is many of the aftermarket suppliers selling NOS, genuine GM or imports seem to specialize in pre-'60 and post'66. More notably, many of the suppliers, not only supply bed parts for the stepside box, but also, sell all of the components require to assemble a completely new stepside box, if not the whole box complete, including '60-'66 stepsides beds.
In the search for bed parts, I would stay with the stepside box due to the availability of parts. Also, if originality is a concern, the VIN with identify the box type, so changing may be undesireable. In closing, SWB and LWB outer wheel tubs/fenders are the fabricated to the same specs and are interchangeable between SWB & LWB step side boxes.
