[SS Tim]

Here are two shots of a C30 stepside with a fabricated rear bumper and steps. Note the wrap on the fenders for protection.
Of course you would probably want a lot less rear overhang that they used.

Also a VFD "race truck" it is C40 based with a custom bed but again the steps are an idea. If you wanted a full running board its possible with planning or just adapt what they did in the step region to your fender lines and keep it flush with the side of the cab as the stockers did.

I have more pictures but many are stock 8 or 9' steps and the others I need to dig about for.

As far as adapting a short bed step bumper you are correct in that the fenders are more forward of the rear sill on a 8" (~7.88") or 9'(~12.33") than a 6.5 but it could be adapted and if you were to replate the top, an excelent starting point for making your own custombumper.
That green GMC is a K2500 8' stepside, not a short bed.