Until you get the details, I wouldn't do anything. Once you get more information like: 1) why he wants to get rid of the 347 (besides just getting a more streetable motor), 2) how many miles are on it, 3) you test drive it, 4) the parts used in the build-up, etc. then you can make a well informed decision.
From first impressions, I would be a little leary of a guy wanting to get rid of his 347. I don't know of many people that spend the bucks building up a nice 347 just to sell it for something 'more streetable'. Ususally, once you get a little more power you want more, not less.
Just be careful and make sure you perform some solid due diligence.
Good luck,
E