Tensile strength on a bolt is the the amout of "pull" it takes to pull ( break ) the bolt apart, its not for shear as with spring hangers, a grade 8 bolt will break before a grade 5 in a shear application, the 5 will stretch and give long before breaking, but with the hanger having a lip under the frame flange it isn't going anywhere gr 5 will work. Neither is as strong as a rivet, especially if it is a hot rivet you may have to drill it completly out if it was hot, to remove it, Spanky may know if GM used a hot rivet.
