Welcome to The 'Bolt, teen48! Lots of good info presented here already, I'll just add one little tidbit. When looking for supplies and materials, check out your local supplier(s) of "large truck" parts. I work for Ryder and all of our cube vans, dock heights, straight trucks are built on the same principles which GL and others already outlined here. The "sleeper" rails are secured to the existing frame rails with body bolt mounts consisting of two flat steel plates with two holes drilled in 'em. These are held together and sandwiched around the frames with two long heavy duty threaded rods with nuts/lockwashers on either end. Sounds like a cool project-why not take pictures while you're hard at it and see if John and Peg would post 'em here for a Tech Tip? Hope you found this helpful. I'm also kinda glad you posted this question-sounds like it might be a good way to finish off an old truck which is missing its bed.
Regards,
Wannabe
