That's pretty much it. Usually some U-bolts or some type of threaded rod clamps are used. On a flat bed we put on one of my International trucks last summer, we laid down a piece of 1x4 board on the frame first, then clamped the bed down. The reason for the wood was, that there were some rivet heads on the topside of the frame rails, such that the steel channel would not fit flush to the frame. By using the 1x4's, we were able to drill holes where these rivets were, and provide a flat surface to clamp the bed to.
(Hope that made sense!!)
