You're definitely not the first one that's happened to! I've never seen one stuck quite that badly before, although I once had to remove the gunky old headbasket from a wireless Sennheiser e835 that was viciously tenacious and required multiple applications of WD-40. The dissimilar metals of the grille (steel) and body (aluminum) corrode over time. I've seen several of those wireless e835 units that did it, but only a couple of stuck SM58s out of the many dozens I've dealt with. But, I don't think any of them were 40 years old, either...
Because the mic body's threads are aluminum, dabbing a few drops of WD-40 around the junction of the grille and body is the ticket. In my many years as a machinist, I've yet to find anything that's a better machining lubricant for aluminum, and it'll also loosen gunked/corroded aluminum threads just as well.
But, another problem now possibly exists: if the pressure of using the wrenches distorted and egg-shaped the perfectly round grille opening, its steel threads will bite into the body's soft aluminum threads and make it even more difficult to remove, possibly damaging them in the process. Always try the WD-40 method first, by dropping it directly onto the area, letting it soak for maybe 30 minutes, then repeating several times.
Years ago (after that one particularly difficult Sennheiser), I got into the habit of touching a WD-40 pen to the threads when I replace or reinstall a grille on a handheld mic, as insurance against it getting stuck. Seems to work great.
