Seeing the photos have me convinced its real. While documentation always helps the value, especially with a 68 Z/28, I'm guessing now that you have it back, so you'll probably never sell it. That alone should be worth paying a premium for it.
I had a chance to buy back my first Camaro two years ago. The guy wanted $12K for it (I had sold it for $1500), and knowing its history, and previous rust issues I dealt with when restoring the body 15 years ago, I new it was a stretch at what he was asking. (Original motor and tranny was gone; he changed the interior from parchment to black; and he painted it powder blue with medium blue runway stripes.) I almost bought it, but instead picked up a much cleaner Camaro for less money. Of course I now have three times that into it ... but then that's a different story.
Had my first Camaro been a Z/28 or an SS instead of just an RS, it would've been much harder to buy it back at any price.
I say keep the 327 in it for now. It makes it a much more dependable "driver".