never meet a spark plug that couldn't stand the heat, so do you mean your cables only? maybe it's just a routing issue, maybe you are running them too close to the exhaust, since you mention concurs can we assume you have stock exhaust or headers, I hear headers run hotter, not sure about the '69 specifically but do they have heat shileds? do the valve covers have wire holder racks, are you using them? just trying to help, cause you can spend the money and get new ones but the same thing can end up happening no?