About the mileage - it's really difficult to know whether the mileage is correct but if the underside is still painted and is intact then it's possible that it does only have 43k. Verification is something you have to address with the seller - he's responsible for proving his claims. If he can't then you're not required to believe him and can either adjust your offering price or walk away.
I don't know the production numbers for cars with AC but they're probably out there somewhere on the internet. It will be a fairly big and expensive project if you want to add it to make it look like a factory installation because starting in 1967 Ford integrated the AC with the heater. This means that for the factory-installed AC, there's a specific heater box, specific heater & a/c controls, vents integrated in the dash, etc.
I'm sure that there were also dealer-installed units that hung under the dash (like the 65-66) as well but I doubt you'd be able to find "production numbers" for those and although the installation might be "correct", it would not look as good as the integrated factory unit.
To be honest, I don't think that having AC is going to add a lot of resale value. The value is more dictated by the body style, engine, transmission and overall condition. But again, if you intend to drive the car often and you live in an area that has a lot of hot humid weather, then it's a good deal if it improves your comfort.
