It's not uncommon for those stress cracks to occur when the front floor pans are rusty or other structural components like the frame or rockers start to fail. The result is that the middle of the car sags down and since the inner fenders are tied to the firewall they can't move. It places a stress on them and the thinnest part with the most weld stresses starts to crack. Right above the shock towers.

Check there first. IMHO.