Long step to short step?

[muddpile]

I have a chance to attain a long step 64-66 GMC for cheap (not a knee knocker cab, but not sure what year past that). It's a 6 cylinder floor shift manual 2wd. The front fenders have mild rust, the rear fenders and bedsides are good, the rockers, floor pans and cab corners are good, and it has minor visor rust (gave it the screwdriver test aroud the windshield).

My question is, how easy is it to cut a long step into a short step? I have the full-blown hotrod Tornado short-fleet right now, but that can only be driven and used in a few applications. A stock-looking dropped short step with a small block under the hood, TH350 and highway gears would make a really nice daily driver. I have a half parted out 77 C10 that would make a great donor vehicle for power steering, brakes, front suspension, diff, trans, etc.

So, here's what I know of that has to be shortened:
-bedsides
-bed floor (likely shot anyways)
-frame

One concern, are the rear fenders the same long to short, and if not, can they be modified?

I think that's it for now, any help at all would be great. Thanks!