On your un-movable seats, besides the flat spotted rollers, if the lower seat rail is bolted down on TOP of the carpeting, the carpet under the bolts will compress and bend the rails slightly. This will lock up the seat adjustment. The stock setup had aluminum spacers that protrude thru holes in the carpet to prevent this, but some larger owners remove the spacers to lower the seats fractionally. Then they need a pry-bar to adjust the seats, or they leave the bolts loose (VERY bad idea!).
Next, there were two seat versions and you may have either. The early seats were three piece with a removable back and a removable headrest. On later seats, the back frame was welded to the seat frame. These frames nearly always crack at the welds, which makes them flexy. Early & late seats are completely interchangeable (but the seat rails are not). The early base is slightly lower plus it is adjustable (a little) for seat rake. This also results in more headroom if you're 6 ft or over.
The DeTomaso Pantera is the world's MOST modified Sports-GT, sort of like the '32 Ford roadster. So if you have something on your machine that you don't care for, if you alter it no one in the club will ostracize you. Just keep all the stock parts for later if you change your mind. BTW, you appear to have a '72-1/2 L model which was a rare transition car (either 571 or 451 made, more or less, out of about 7200. So it has many pre-L (early) parts and some L (later upgrade) parts straight from the factory. Don't get confused when you see stuff on other "stock" Panteras that you don't have.