'72 barn find (kinda)

[TxGoats]

I looked at a '72 C10 yesterday. I'm thinking about buying it, it has been sitting for 6-7 years. The gentleman that owns it is elderly and when he lost his sight he parked it in the barn.
The truck is totally rust free, restoration was about 15 years ago, crate motor, cab corners and rockers redone so no rust. 350 automatic with factory tilt wheel and dash tach. The interior is immaculate and the rest of the truck is under 1/2" of dust and muddobber nests.
The kicker is, it has an '89 sportside bed on it. Not my favorite but the cab is so nice I'm considering it.
I don't want to insult the gentleman with too low an offer, any ideas?
It will need a complete detail cleaning, tires, hoses and belts. What should I offer the guy?