Fool proof easy & quick way..... Use a set of vise grips on the square nut. Clamp it down TIGHT & rock the nut back & forth, it will get hot & break right off.
If you are a masochist, you can spend all day backing those square nuts out over 50 years of crud on the carriage bolts, dont forget to hammer a 13mm socket on those square nuts ,of course the carrige nut will start to spin along with the nut as soon as you break it free. What fun !! Better yet, lay on the ground & blast the square nuts off with the torch. You will be peppered by molten metal at that angle. Even more fun !!!
Believe it or not, I have seen some knuckleheads [ no offense to the Harley Davidson dudes ] actually cut the running board supports off the frame with the running boards still firmly attached. Thats the funnest of all !!!
