I have u-bolts that are custom made and are the answer to having to run those cheapies from your local parts store. I have 1/2" grade 5 and also fat 9/16" grade 8. You're definitely going to pay more than you would for china specials, but as soon as you hold one you'll be glad you did. Square or round, I have them both. The threads aren't cut as deep, which means there is no play in the nut. Even custom lengths beyond the standard 10" found at parts stores. This is one area you definitely don't want to cheap out on. I also have the strongest steel blocks on the market. PM for details.
*all come with grade 8 hardware*
1/2" grade 5 USA
9/16"" grade 8 USA