yes, the panel specific things are added to the same frame as the pickup uses, the rear frame horns are to provide the bumper mounts because the body extends further out than the pickup bed - all could be welded I suppose, but I believe the holes will be in the frame and grade 8 bolts will do the job, the hard part is getting the rivets off
saves a bunch of work to have a better set of doors, but chain? would that be the common farm replacement for the top
check arms? you should try to find them if they're missing .... and the spring is the wireway for the tail light wire, a screen door spring works there

Bill