I don't know the answer to your question.
I'm thinking either your front or your rear has already been changed. I'd suspect the rear, but don't really know. I believe Chevy used 6-lug wheels on 3/4-ton trucks at about the same your truck was built so I wouldn't be surprised if the same hubs were used on 1-ton trucks.
That's all just guessing so take it for what its worth...
If you want to carry a single spare and have it match the rear you now have, you'll need to be sure what front hubs could take the same wheels.
Do you know what the rear is?
