Grade 8 nuts are available by the thousands on Ebay, and also at any good hardware store. What's with the obsession about replacing good-condition U bolts with no thread damage? Those are fine threads, either 5/16-24 or maybe 3/8-24. What size wrench fits the nut? If it's a 1/2" wrench, the thread is 5/16. If the wrench size is 9/16", you need a 3/8" nut. Just replace the nuts, not the U bolts!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!