Nick, I read somewhere that the inner wheelwells on the 15 cwts had to be different because the 4x2s would turn sharper so need more clearance. I don't know if the 8 cwts were this way too. Most of the cab panels should be the same.
On another thread I posted a pic of the floor panels in my C8 with a different treadplate pattern on the passenger side panel. Well, the C15 has exactly the same mix of panels. Check yours and see if they're like that too.
C8 data plate
Colin, checking them out sounds like a good idea.
I have the floor panels off of three Cab 11 Chevs and they're all the same. The panel under the seats and the drivers side panel are dot and dash treadplate and the passengers side plate is all dashes.