I have been burning standard regular gas, with an occasional tank of hi-test around inspection time to make the Check Engine light go out. I guess I have only stopped up the plate inside the 'Cat' and have not driven it hard enough to burn-out the plate. Should have been using hi-test more often, I guess.

Based on other comments and a review of photos here on the site, it looks like they are welded-in. So, I will have to find a muffler shop to do the work.