Talk to Cobravalley about new rear shocks. I don't know if your originals either can or are worth rebuilding.
Cobra Valley's SHOCK & SPRING Suspension UPGRADE
It sounds like soft old shocks, worn suspension and the 17" in combo are what's loosening your fillings. Check the rear shock mounts, number of bolts, crush sleeves are all as they should be on the back end. Front end I really like and feel a huge difference from their ball joint upgrade.
Cobra Valley's R/T Super High Performance Ball Joints A good local trimmer should be able to re-web and or re-cushion your seats. Don't let him overstuff them or you'll be jammed up against the underneath of your steering wheel with your knees.