I have a 1970 Chevy C10 and the Bed appears to be a Dodge 1961 and up vintage. It is 54" wide and has the 3rd (middle) tailgate hinge (makes it no earlier than 1961. When I measure the truck bed length inside I get 78" but most web sites list it as 76". Any insight why mine is 78? Personally, I don't think the truck knows what it is.

 

late 70's bed.... The left fender kind of throws me off. Not to sure what years/model that fit... But it looks period correct... Looks like a 6' bed.

 

I picked up a 1978 Dodge Truck sales literature and it showed the measurements of the bed as 78 by 54 so I know that the bed is stock size. That answers one question - as I see some web sites saying it is 76" long.

 

I guess it kind of depends on how the others are measuring the bed itself?
But it sounds like you got a 6-1/2" bed out of the deal.