Google is your friend. Go to google.com and query "flywheel runout" if you use the quotes, you get the exact phrase. Then, add Chevy to your search..... you get hundreds of hits on this.

Measuring flywheel runout at the flywheel face is a tough one....cant really do it on the engine unless you can eliminate crank end play. Using a lathe is easier, using a surface plate might work if you can set up a plane for the flywheel mounting flange to sit on.