Leaf Spring u bolt nut problem !!!

Its called a scored nut. Look at it closely and you'll see a tiny square stamping in the middle of 2 opposing flats. Get the nuts hot enough fast enough and they will zip on/off with a impact.
Not many types of nuts would work here since they're torqued to only somewhere around 60 ft lbs ( I forget ), too much part movement .
The U-bolts aren't supposed to be reused but many have and will.