Yes, are are not only right in a morale sense to use the sheetmetal and not the whole shell, to save your Dad's Z, but also in a legal sense as well. Swapping a VIN tag to a new shell would be a REALLY bad (read that as illegal) idea.
You are doing a great job. Keep posting the progress photos. I need to see that someone else is as crazy as me!
P.S. When you get around to stripping that shell, I could use the driver's side, emergency brake cable metal cover. For some reason they forgot to install it on my car at the factory:
Here's a photo of the location, and of the part itself (this is the passenger side version, though):
