The copyright you are violating is not John's. You've have posted .pdfs of the actual magazine pages and that's copyrighted by the publisher, Amos Automotive Publishing. While John may have given you permission to use his text, what he couldn't grant you is permission to repost the magazine's pages.
Links are not copyright violations. What's a violation is reposting the actual magazine pages or posting of an author's text on your site without permission from the copyright owner. You have a ton of CE's copyrighted material on your site. I personally, have not stake in that but the magazine publisher may feel it's copyright is being violated. The proper thing to do is contact Amos Automotive Publishing and ask for permission to reproduce pages out of "Corvette Enthuaist" for your web site.