I can't help you with the size,(I can't remember) but IMO, do go for the grade 8 bolts.
My story: I found the irritating rattle, on my 98, after replacing several different parts. It was caused by a very loose fit where the bolts go through bottom shock mounting holes.
I bought new front shocks, thinking the holes were wobbled? out, and too large for the bolts. As it turned out, the bolts were worn worse than the shock holes. The bolts were about 1/8" smaller in the middle, than on each end.
The bolts were metric,and I could not find any grade 8s that big locally. (About 1/2 or 9/16"? if they were ASE. ) I ended up using a grade ? for temp fix, and I'll buy grade 8s, and fix this spring, when warmer.
So if you find a source for those grade 8 bolts, I would appreciate a response.
TIA, Ray
PS. Make sure to check your existing shocks, that the holes are not worn too big.