I have looked at a mix of several different 66 & 67 Chevelles over the past few months (including the Moultrie show in GA) and you would be lucky to find one in decent shape for less than $4,000-5,000.
Just from the pics you have I would guess he is asking at around $7,000 regardless of the fact it is not a stock setup. If your looking for a stock trim Chevelle fid one that is already done and save some money. If not, I would say if it runs and has a good body on it, and is less than $7k it's a good investment.
Just look around on Ebay, or Racingjunk.com and you can get an idea of what people are getting for their Chevelles.