Here`s what I do. First make sure your using the correct bolts, they should be knurled and have a shoulder on them. There should be no washers on the bolts. There are grooves in the starter than the bolts shoulders fall into, make sure these grooves are clean so the bolt shoulder fits in squarely. Reinstall the starter, and torque the bolts to 35 ft. lbs.
Next go over your grounds. I would run the ground from the battery to the block and another ground from the block to the frame. I`ve used this method on many cars and so far, it`s worked every time. Improving the grounds helps the electrical system greatly, not only are the lights brighter but the electrical system don`t have to work as hard so it lasts longer. Also check the factory grounds coming off the firewall, make sure they are present and connected.