Personally I don't see an issue with using regular grade 8 bolts.
The shoulder on the original is a machined fit verses the unthreaded portion of a regular bolt (not to be confused with a shoulder bolt). Is there potential for the caliper to move? Maybe, but once it moves that minute amount it isn't going to be going back and forth.
I only had two, on each side, couple of grade 8 in the other.