Here is a link to my 9ft bed I built. The cross sills were original (except one), the bedsides and front panel are to specifications, the tailgate was Mar-K, pine bed wood, stingless hardware.
The photo bucket links don’t work, but the google ones should. It might help you visualize the “underside” stuff.https://www.stovebolt.com/ubbthreads/ubbthreads.php/topics/1117926/6.html
There are some pictures of the bed assembled further on as well. I wish I had never used photo bucket. I lost so many pictures of build details because of that.