Personally, if you are going to the trouble of attempting to kit bash two kits to make a new trailer, I think it would be easier and your results would be more accurate from a modern subject point of view to build it from scratch. A flat bed is among the easiest trailer to build. There have been several excellent scratch built trailers on this forum and you will learn a whole lot about trailers, acquire new modeling skills and have the satisfaction of knowing you can tackle any model issue. Dont be afraid to ask.