Where has it been parked the last 60 years? If its been in an Eastern Washington garage it might be in spectacular condition and easily worth that money. If its been under a tree in Western (Wetter) Washington it could be low miles and all original and hardly worth scrap metal.
As for how to tell if its original or not, there is no registry of serial numbers and parts numbers that will confirm its matching number original. You can check the serial and casting numbers on major components, but all you are really doing it confirming they could be original.
The paperwork would give a clue. If the ownership can be traced back to new and is all people the seller knows, the claim in more credible than if they bought it from some farmer and don't remember his name or any more about the trucks history.
I'd ask what all-original means to this seller. Is it actually all the original parts it left the factory with, or has it been restored with the correct parts to be original.
If I really cared about original parts I'd take lots of pictures in good light, inside, outside and underneath, and post them somewhere that people from sites like this can see and comment on. Various people will spot details that can be very telling and easily missed in a 30 minute look at the truck.
I might even brag about how original it was and in that way challenge people to find things that prove it isn't.
That's my two cents.
