The fender tag and build sheets are two very important parts of the pedigree for those mopars to justify pricing. Without them, it is just a vin. However, the vin can tell you something about it. If he has given that to you, try running it through a decoder. It will verify if its a real Road Runner, engine size, and vin sequence. There should be a matching sequence of the last 6 vin digits on the radiator core support and along the drivers side trunk rail. If any of these don't match, you've got issues.
http://www.stockmopar.com/mopar-vin-decoder.php
I notice in the pics there are a few other mopars scattered around the property. That means this guy
should know the value of these documentable items and you may be able to talk him down to a decent price without them.
Something I've always like about that model was the top of the quarter panel line. I think it would lend its self very well to an eliptical insert to widen it out for a massive tire while still looking
almost stock. Only way to notice it would be to park it next to an unmodified version.