Great looking car. The paint looks decent from your pics. If you painted it the original color, I don't think you would re-coupe your costs right away with the current market. So investment wise i would "stick" and enjoy the car as is, and switch out small items like spark plug wires, breather cap, figure out the antenna if it was not a factory radio delete car. You ran most of the numbers, so any other small items I would get to make it that much more correct. (Tower clamps etc.)
As far as your question regarding Z/28 vs Big block cars out there. There are so many fakes of both I would not be able to fathom a guess of what is really left by what you see at shows and online. I would guess there are equal number of both left, but if the motor has been removed from these cars, they could have been restored differently than they left the factory.