I agree with old5.0....
Back then people used to frequently call the 5.7 Liter option in the Iroc a "Corvette motor"... as earlier Iroc's were only available with the 5.0 Liter 305. Granted the 5.7 iron head motor in the Iroc wasn't even the same one as in the Corvette, but it didn't stop everyone who ordered the 5.7 from calling it a Corvette engine. Heck, my own dad had a 5.7 L98 Iroc and called it a corvette motor.
My guess is that it had the 5.7 TPI option and somewhere along the line the story changed from Corvette motor to ZR-1 motor. The ZR-1 didn't come out til late '89 anyhow, so it would be impossible for a 1987 Iroc to have one factory installed even if it was a direct bolt-in...it was built 2 years before the first ZR-1 vettes ever rolled off the line.