Big block short box identification.

[Chrome8]

I'm trying to figure out if a truck that I'm looking at is truly a short box big block truck from the factory. It's being represented that way, and the glovebox door says big block 115.0 inch wheel base but I would like to know if there is any other way to tell what it left the factory as. The truck has been restored so they could have printed a new glovebox label or just used a different glovebox door. I heard from a friend of mine that the firewall is different on a Big Block truck but I don't know if he knows what he's talking about or not. Any help would be appreciated. It makes quite a bit of difference in what these trucks are worth right?