Original owner after 44 years. Looks spectacular! https://seattle.craigslist.org/see/cto/d/butte-1975-pontiac-grand-prix-lj/6912424861.html
Nice looking Car, rare to see a split bench seat in a 75, they became very common in 76/77 but mostly bucket seats before that. No mention if it is 400 or 455 powered (the only engine choices) so I assume a 400. Still very nice, but very ambitious price.
If it has a 350 its not the original motor, 67-75 the 400 was the base engine in a GP, 76 it became a 350, 77 a 301, and 78-86 a 231 V6.
It's also possible he's just guessing, and flat wrong. It would have to be investigated to see if it's numbers-matching or not.
Honeycombs and red velour ? Any man driving that car should be required to grow mutton chops and wear bell bottoms. Good luck on $ 19k. Just don't see the emotional pull for that car. Check out the mid 60's GPs for a reason to spend that kind of money.