What I think is that the cab forward is mostly original, but the bed was swapped for an 80's bed as when it was painted and bondo'd up as the original bed was too hashed for bondo (and likely good shape 80's beds were easy to acquire at the time)
For reference, here is parts of my 79's original bed:
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Then an 85 bed side. The 85 has some plastic plugs up at the front whereas the 79 does not. not sure when they added them but I say with confidence that it was not until the 80's
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From your other thread you can see the plugs
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On the tailgate itself the trunnion a 79 should be spot welded on, in the 80's they went to bolt on. Along with a 79 GMC should have the GMC embossed in the center of the tailgate, in the 80's it was off set to the left.
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You can see the clear coat stone gaurds that AFAIK was never on any 73-80 truck. I do know for certain that by the mid 80's they were standard on trucks.
@Keith Seymore might be able to shed more information on this as he was there during this era.
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So if I was you I'd pull the trim panel off the tailgate and see whats underneath. If you are worried about a cab swap, I would look at the rivets holding the VIN tag on, if they are flower shaped its the OG cab, if not then its been tampered with. Happen to have any underhood pictures with a good view of the firewall?