I think you are right about seeing the bolts. However I figured, they came with the kit so why not. SS is very hard and has a very high shear strength. It is comparable to grade 5 or higher. I shouldn't need anything more for the starter. I can't remember at the momment, but I believe my bolts were larger than 5/16 as well.
Edited: Nope, 5/16 is the bolt size.
--Mike