Barn find - Should I be nervous??

[Derby]

Hey everybody. I know there are lots of you with tons of experience so I need to pick your brains. I have found a 1970 C10 lwb with about 78,000 miles. It had gone through a restoration over 20 years ago. It was bought at an auction and has basically sat in a shed ever since. The VIN shows it should have a straight 6 but was at sometime swapped with a 327. The engine and brakes were recently gone through. Small amount of surface rust on bottom of the right door, otherwise clean and straight.

Question 1 - What "hidden" issues may this truck have sitting so long?

Question 2 - Is $3,500 a reasonable price to pay?

Thanks so much!