https://akroncanton.craigslist.org/cto/d/waynesburg-1984-chevy-caprice-station/7118554231.html Buick front seat?
That is a Buick front seat, no doubt. It doesn't detract from the value in my opinion as those seats are some of the best riding seats ever made, it also is the right color to match the rest of the interior. I was about to say "they didn't come with TH400s..." and then I saw the engine compartment and it's obviously been upgraded. I'd prefer this drivetrain over the stock 305\307 with 200-4R for longevity. The lack of the air conditioning equipment will turn off some buyers but he may get enough that don't mind.
Going through some on-line brochures, the front seat upholstery and design seems to match that of a 1980 Olds 98 Regency.