The Great Superman Book is copyright Michael L. Fleisher, so it's really his opinion that matters. This book came out approximately 30 years ago and I have no idea where Mr. Fleisher is hanging his hat these days. I cannot even begin to speculate what his opinion on this project would be.
My experience with DC has been that if they don't contact us, then what we're doing is OK. As soon as anyone does anything that they don't like, they will make their opinion known, and fast. And it's not like they're going to sue anyone, their first step has always been to just ask us to stop. Sort of a, "you don't know unless you try it" approach. Not very formal, but it's gotten us this far.
I would say it's OK to put the articles up, and then if DC or Fleisher complains down the road, we re-write them or remove whatever the offending content is.