imo, and in my experience, the grade of the bolt is only as relevant as the quality of the metals and threads into which it is inserted. Having a $10 bolt run into 5 cent threads of poor quality metal, gets you what exactly? Just saying.
Actually.. there is way more to it than that.
If you are only looking at clamping power, and you have bad cast iron, and say, fine threads.. or small bolt diameter, then yes.. the bolt can be pulled striahgt up and out, however.. not all pulls are straight up and out.. side load a bolt and then put upward stress on it.. or uneven upward stress and a side load... a soft bolt can deform or shear. that side laod actually ADDS to the force needed to pull that bolt out of the casting.
While a grade 2 bolt may be a good match for cr@p iron castings.. it's head may cock to one side, shear off or otherwise deform actually adding to the problem, vs a gr5 bolt that's a lil harder. ( and a lil harder to drill out too.. )
I think the course of action to measure a known good bolt and hole is a good one.. who knows.. maybee they were m10 and already stripped, and have already been shodilly repaired by tapping 7/16 threads which were not deep due to the metal already removed, and then the wrong bolt slapped in on top of inadequate threads.
in the end.. some sort of OS on the hole will be needed.. either drill and tap for some sort of insert to get back to original or a standard size.. or simply OS with OS fasteners..or OS and plugged and redrilled standard.. heck... might be a good application for brazing,drilling,tapping... can't be -ANY- worse than what's there already..
soundguy