yes yes yes. all parts to make a new stepside bed ARE available for purchase aftermarket (at a high cost). a stepside and a fleetside ARE interchangeable. once you have all parts in front of you you can assemble the bed and install it in an afternoon/day. go for it. you are making this out to be much bigger than it really is. as i linked up there you can purchase a COMPLETE bed. if you dont want fiberglass you can purchase all parts to make the original style stepside bed from mar-k, Classic parts of america(formerly chevyduty) and LMC truck. once the bed is on if you dont like the custom cab look take the trim off. good luck and let us know how it comes out.
