I'd like to hear more about this seat/storage deal....Idaho??
Make a rear cage in your truck with mounting tabs so that you can have individual substructures with separate seats or racks or speaker boxes? The mounts would make it so you would have a totally modular system that everything would be solid and stable. You could use either rear seat or both, or one speker box and a seat, or two racks? Whatever you wanted. The Bracket mounts would be essentially control arm mounts with the crossmember setup having polyurathane bushings to help isolate the ride of the seats. You could use a quick release pin style.