Good moring:
It's hard to say the size of the bolts, but they are special bolt that are heat treated for streanth. If the parts store don't know the size you can always goto GM and pay an arm and a leg.

OR!!

goto a wreckers (recyclers), you'll get them cheap!!

you can make sure they for starter bolts by looking on the neck of the bolt, it will have a chris-cross paturn going around it. I hope I have helped you, good luck with your search.