For your striped holddown bolt, if its the long style that bolts on top of the battery with the long stem that hooks under the battery, you can use a hacksaw and replace the bolt after you cut it. If its the single bolt with the plastic block at the bottom, that is going to be a lot more challenging. Are you sure its striped or is it just nasty from acid? Spray it with wd-40 or liquid wrench and let it soak for a couple of hours.