The big Pontiacs were some of the best cars made in their time.
They were (and are) great family vehicles that were fun to drive and take trips with 5 or 6 people in. They are underappreciated these days by a lot of people who have no idea what a nice car really is besides a fancy nameplate on an average (or even mediocre) car. Dont buy any car as a potential "investment". A buyer should buy these cars because they "like" the car. These high priced "investors" have ruined the old car hobby to some extent by driving up prices on certain cars that they "believe" in their minds, to be a nicer old car than another one they dont think is worth anything. All older cars have their vices and virtues, one may be better in certain categories than others and vice versa. As an extreme example- Is what is basically a Plymouth Satellite (Road Runner) with the RB (Hemi) engine really worth 2 million dollars, while an actual Satellite with a Slant six not worth hardly anything and someone sells it for $2500 after sitting on it for a couple of years?
Sidesteeping all the original "restored" crap such a "reproducing" the original chalk marks and other nonsense, a person could buy the cheaper-(much cheaper) car , put in a 528 crate hemi, restore the body, redo the interior, update the chassis, and blow away the "original" Barrett Jackson crap in all functional categories, as well as have more fun to boot. And have enough money left over to buy a mansion and a first class trip to Hawaii.
The same holds true for these big Pontiacs. Buy the one you like, do the body and interior real nice, add all the creature comforts you can that could have came with the car, while not detracting from the appearance. Then put in a nice Pontiac engine with well thought out components and an overdrive, then hit the open road and enjoy. The enjoyment is "the investment"
Dont plan on recouping the money outlay and youll be fine.