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The frame both in front, and in back of the rear axle will need to be shortened. Drive shaft will need to be cut, and you will need a short bed fuel tank as well.
Yes an 8' step side will bolt directly onto your truck. No frame, or drive sahft cutting. Only thing you would need is the step side gas tank filler neck.
I hate to say it but if you don't think you can or have to tools to do it right, might as well buy a short bed frame and bed, driveshaft and tank and put your cab and front clip on the short bed frame.
You know it looks like you could make an 8 ft. step pretty easy from a 6 footer.