The older generation 454 had the stamped steel valve covers on it, like the engine you pictured from this truck. All the 454/7.4 engines I've seen in the 400 trucks from the factory, have the cast aluminum valve covers, usually painted black with the tops of the fins machined off so they're silver. And the later engines have some smog control lines that are absent on the older ones.
So it seems to me, that some previous owner pulled the 6.5 when they couldn't get it fixed, and dropped a random "454" in its place.
With enough time,$$ and dedication this could still be a very capable combination. You'll have to decide if you like the rest of the truck enough to deal with it.....