If the vin is accurately listed, it's missing a letter, and has one too many numbers correct for a 77' correct.
Correct.
U15GLA00000 is the correct or expected VIN setup.
U15 series, G for 302, L for Michigan plant, then a single letter (A-D) and 5 digits for the production order/number.
Whoever is going to look at it, be sure they get shots of the drivers door striker and glove box (if they still exist). If not, you might be down to just the frame and some states don't allow for 1 VIN locale to be enough to title it [even if you do claim it]. Which is why he's only got a BoS.
For reference when/if you get a shot of the full tag:
Color Code B - Dk Jade Metallic [as claimed in the ad], Trans code G for auto, DSO for ATL was 21.
But taking a second pass:
The front fenders have also been replaced (or the side markers filled).
Engine bay is blacked (should be body) and was probably done during the respray (along with the fiber hood). So I'd pay close attention to the front end for damage since the clip might have been replaced. And they mention new rad/pitman arm/carb/ign and that's not stock wiring.