Before I bought a set of leather H3 seats, I bought a set of '06 Grand Prix seats, and just removed the stock GP brackets, which were powered seats (still have 'em, if someone needs motors or the entire GP lower seat mounting mechanism for both), and was left w/the seat pans, which just needed a couple of holes drilled for two mounting bolts on each seat pan to adapt them to the stock 355 rails. I dunno how-similar the GTO seats would be, but I would think pretty-close. The stock GP seat slider & electrical operation system is too-wide for the 355 floorpan.
Looks like you did OK on your fitment, the main thing is to have them well-secured in the event of an accident, and also having the airbag seat cushion mechanism switched-over so that will still work. Collapsing seats were one of the things that hurt a lot of people before cars were built safer. The old VW 'bugs' were notorious for having the seats collapse and contribute to passenger injuries, partially due to the seats tearing-loose from corroded seat pans due to salt damage, and a lack of adequate strength in the stock seat design.