Chevy Or GMC- What do I own?

[68chevyfuelie]

Does anyone out there know how to break down the VIN? I own a junkyard-built '68 Chevy/GMC, it has Chevy badging on it now. It has a 12 bolt rear end, rear leaf, both rear shocks point toward the front of the truck. It originally was a 6 cylinder, 3 on the tree. There is no build sticker, just a VIN on the door post. VIN is CS10C PB27587, underneath that 'DD'.
VIN plate says 'GMC Truck Max GVW 5000'. Under that is a plate that says "General Motors Corp certifies this truck to meet all safety requirements". Any help is greatly appreciated.