I did the same thing. Used grade 8 bolts. The thing does not rattle. The grade 8 bolts are just fine, If you manage to pull a manuver that breaks the bolts, you will have lost the boat off the trailer. But I gues if you pull your boat through the Baja 1000, you may consider a less brittle bolt.
http://www.hotboatpics.com/pics/data/500/5859pivot.jpg
http://www.hotboatpics.com/pics/data...pivotworks.jpg
If you can find the website for Pivot Hitch, they have improved the design, and I would go with the new design.