I realize I left out the 2-piece/1-piece windshields, cowl vent doors, etc, etc, etc, etc, etc, etc. But as I said originally, I was only going to point out a few differences that were easy to see. Plus, it would take more space than is available here to point out all the 47-55 differences.
The Cameo bed was NOT an overlay, it was a total, completely different bed made mostly of fiberglass. The only thing the Cameo bed shared with the stepside bed is that it was installed on the frame behind the cab.
To the best of my knowledge, the STEEL fleetside bed was introduced in 59. I think the only straight side bed in 58 was the Cameo.
The two-tone colors were only 57-58. The 55-6 Cameos had a "plain" bed and the cab/bed were the same color. Whereas, the 57-58 Cameos had the added chrome trim on the bed and inbetween the two chrome strips along the sides of the bed, they were painted with a contrasting color. All the Chevy Cameos came with the custom cab which had the big, wrap around rear window. But the GMC Cameos came with the standard rear window as well as the big, wrap around rear window.
If the Chevy Cameos had a V8, it was a Chevy small block. But I believe the GMC Cameos with a V8 got either an Olds or Buick engine.
You're going to have to locate a very knowledgeable GMC Cameo person to nail that one down solid.