Chris, it's looking good, but before we can confirm, we need a LOT more information.
1. What's the VIN?
2. Post a higher resolution picture of the trim tag
3. The axle appears to be stamped 0614 (June 14th), but what's the casting date on the center section (the "pumpkin")? BW was used in 1968 AND 1969, so the date would confirm if it's original to that car.
4. The Muncie Speedometer hole, rear bumper guards, 4 leaf leaf springs, 2 piece disk brake rotors, correct booster with wt cylinder, etc... were NOT limited to just the Z28, so none of this is conclusive.
5. Not all leaf springs will have the GMU casting. This casting was done every 10 to 15 feet, so it just depends on where the spring happened to be cut. There are a lot of cars out there with original springs that DON'T have the GMU casting, so again, inconclusive.
6. The paint sounds promising so can you show some better pictures of that?
7. You stated that you have no documentation, so how do you know the car came from Bill Conner Chevrolet? Not sure if they're still in business, but if they are, (Zone 25, Dealer 95), it would be worth a few phone calls.
Ed