Proper high torque mini starter bolts are knurled below the threaded area. If the GM bolts are not like that, you can get the Hitachi bolts which are like that. To get the proper length, I would push a wire into the blind hole and mark the wire at the mounting surface, then measure the depth of the mounting plate on the starter. Add those two dimensions together and subtract the amount you want to seat short of bottoming out in the blind hole.
I am not sure about the length but this is what they should look like:
http://www.ebay.com/itm/Chevy-Buick-...ke%3AChevrolet