Just because they broke doesn't mean they weren't strong enough...they could have been loose or other bolts may be loose elsewhere causing these 2 to be overloaded or the mounting holes are worn causing excess movement in the mount.
Also if the others are grade 5, going to grade 8 does not necessarily make them "stronger", while they have higher tensile(straight pull) strength they are are more "brittle" in shear load. Sometime (although rarely) going down a grade can work out better as the bolts have more give (stretch) to them.
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2007 Haulmark 3329DS, Freightliner Coronado Chassis, 515hp Detroit, Meritor Freedomline 12speed Autoshift, 10kw Generator, In-motion Satellite.
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