Okay, thanks for the info! I knew the floorpan changed for the cats but didn't realize it affected the mounting of the seats.
I'm thinking I'll look around for a set of trashed '71 - '74 seats and just scavenge the brackets off them. If I can't find any, then I'll buy the cheaper Corbeau brackets and make spacers as necessary.
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'72 Formula 400 Lucerne Blue, Blue Deluxe interior - My first car!
'73 Firebird 400/4-speed Black on Black, mix & match.