An update, based on the feedback from Milkman and Walt, I replaced the bolts with 7/16 x 1 ½ " bolts, grade 8, with self locking nuts and two flat washers, Sears Hardware had a great selection. I had to drill the plate on the tractor to get the 7/16 bolt to fit. Other then that, all 4 bolts worked great. The movement on the 3 pt is considerably less then using the same hardware from the turnbuckle. I am ready for the 6 inches of snow tonight with the new snow blower. Thanks guys for the help.
Tom