I doubt any production ZL1 engine got near 575 hp. We worked with Bill Porterfield back in the '90s on a ZL1 dyno comparison. With a good tune up, open headers, no AIR pump, air cleaner or alternator it did 523 hp.
http://www.camaros.org/copo.shtml
Camaro High Performance has rebuilt several. With racing oil and modern rings, they see 550+hp.