Oops, forgot to mention. Clean the threads well on both 'nut' and proposed replacement bolt. Don't use a wrench. Turn it in by hand to test fit and test 'jiggle' for loose threads. If you have to use a wrench, it's an incorrect thread.
Lots of times in cases like this, I break down and order from dealer to be certain of fit. Many times the part number in the parts book (online at Kubota.Com) is something a stocking dealer will have on hand. I've found dealer bolts are about twice cost of what Tractor Supply might be asking.