Subzali, check me.
In general a grade 5 bolt will begin to deform and potentially hit its ultimate before the same size grade 8 begins to deform. The only reason in my opinion to use grade 5 is cost. If you are manufacturing trucks and were sourcing 25 million bolts, grade 8 being a few cents more each makes a difference and would be an unnecessary cost if grade 5 is sufficient. What difference does it make to us if 10 bolts cost $10 or $12?
I'd use grade 8 (or like Jeff mentions DIN class 10.9, I prefer my bolt heads to use the correct 10/12/14/17/19mm spanner progression) and the largest that fits at all critical points. The problem as I see it is you can't guarantee that all the bolts are equally loaded and so you must assume some subset of the total is doing the majority of the work. I'd go ahead and chase holes to 12mm or 1/2" with the bumper fitted to make the connections as snug as possible.
A 1/2" carbon steel grade 8 (150ksi) in single shear has a yield (assuming 60% of ultimate) of about 17,660 lbf and the same in grade 5 would be 14,130 lbf. For comparison the same in 3/8" would be 9,935 lbf and 7,948 lbf. Those numbers are per fastener, 10mm and 12mm bolts would fall roughly between those numbers, 10mm being ever so slightly higher than 3/8" and 12mm being slightly lower than 1/2". So the safe assumption is to analyze for just one bolt and then you have a whole lot of redundancy and in practice no bolt ever comes close to being stressed.
"Silent enim leges inter arma." -- Cicero