340 advice

I am looking for some advice on a situation that I have. I have the opportunity to buy a 71 dart with a 340 in it. Now here is the issue, the motor was built back in the mid 80's and has (according to the owner appox 50 miles on it). He also claims that it has been 5-10 yrs since it was ran and it has basically been sitting the whole time. I know what my gut is telling me but I thought I would ask anyways. Would the right thing be tear it down and go through it or try to get it running the way it sits and hope for the best. I was able to turn the motor over with a breaker bar and it was free. I was curious if anyone had a similar situation and how they handled it. Any thought or advice will be greatly appreciated.
Thanks
Dan