I took my rear seat frame and old fill material to a local shop to get their opinion on whether to replace the foam and they said just to use it the way it was, which I did. It came out pretty good but after seeing how much better I like the front seats after I added some extra fibre-fill to them, I wish I would have added a bit of extra filler to the rear bench as well. maybe some of my pics from the job will help you.
http://www3.telus.net/68bird/interior_work5.htm