Angela, Here is another option.
When I worked in the aircraft industry there is a self aligning washer that we used to use when the surface was not perpendicular to the bolt hole. Here is a couple of links for those. The 8mm will fit a 5/16" bolt and the 10mm will fit the 3/8" bolt.
http://www.grainger.com/Grainger/wwg/itemDetailsRender.shtml?ItemId=1613562051
http://www.grainger.com/Grainger/wwg/itemDetailsRender.shtml?ItemId=1613562053