The rear end is a 12 bolt. The frame is actually out of a chevy. The frame that came with the truck when i bought it was torn down to nothing, with no suspension components or mounts whatsoever. Plus someone at one point torched big chunks out of some pretty obvious parts of the frame. Sense we had already bought a 69 chevy to get the front suspension, we decided that it would be easier to use the chevy axle (not making it true to gmc, because gmc 2 wheel drives always had leaf springs in the rear, and chevy 2 wheel drives always have coil springs in the rear) rather than trying to track down a gmc frame that had the leaf spring mounts. Not what i originally wanted, but most people wouldn't know the difference, so i'm not to worried about it.
Here is a picture of the truck we got for the frame