Damn dude...you've got a lot of work done! Nice write-up too...

Ok, as for the drivetrain on the GMC...consider it trashed. I would not reuse or try to fix anything. There should be enough flex in the driveline to where the rear end should still be good. I would take the motor, motor mounts, tranny, and driveshaft out of the GMC and replace it with the Chevy stuff. Of course you will need to get a new CCSB driveshaft for the GMC. I would save the motor and tranny from the GMC to use as your own parts stock.

From the looks of the frame, you are good--just swap over everything from the Chevy that you take off the GMC. It should be a pretty straight forward swap.

Good work--keep us updated!