First and foremost, a truck is only original once. A truck with a repaint, replaced tailgate and a rebuilt engine is not "original". It can not be put back to original. It can be returned to authentic or "stock" condition.
That being said, it sounds like great find. The fair price is the price where both the buyer and seller are happy. Sight unseen, there's no way to give a reliable figure. From the description I'd say it's worth a minimum of $1000-1500. It will probably take another $5000 to get it running reliably and another $5000-10,000 on top of that to get it "right".
For GMC reference material and pics go to
http://www.oldgmctrucks.com/
