yea long bed steps sides I've only seen like 2 or 3 in my life and online only but a guy local has a complete stepside bed long bed for sale.
honestly I think a stepside would be easy to chop down, it's flat on both sides, cut the front and trim the step, then cut the back, then bolt on the fender, leave that area all in tact.
a fleetside bed tapers up in the back making it a real pain and the front is flat and easy. I saw a guy on another forum do a fleetside bed.
I truely think, haven't seen a stepside in person to compare that closely, but I think it'd be tons easier. You can move and redrill the cross sills to make them work as well.
The frame seems pretty easy, flat part behind the cab like 12 or 14 inches there and then move the back crossmember up? and cut off like 4 inches on the back of the frame from what I've seen. it's also on another forum, both on the same one I think.