I personally don't use any Hagerty, etc, price guide. Perhaps that's due to 25+ years of being into C-bodies, and values (to me) are what I'd be willing to pay, based on what is there in a car, what it needs, and what else might be out there as an alternative purchase. I am sometimes low vs the market, but that's OK with me. I've never bought a car wherein I regretted what I paid.
68 is the transition year of VINs on drivetrain, IIRC. Easy to verify the block's year when no AC like this one, though.
IMO, #s matching is nice but on a 'vanilla' car the condition of the car is more important to the valuation.
A car born with an HP drivetrain, or maybe loaded with options, would be worth more if orig drivetrain, of course.
On this car, one thing I'd wanna know - does he have the bottom of the back seat? That would be an annoyance to find, ship, etc. 'Minor' things like that sometimes turn me off completely.