I want to lower my car. The rear spring has 6 1/2"or 7" bolts now. Want to go to a 10" bolt. My stock bolt is a 9/16" grade 8 bolt. Can I go with a 1/2" grade 8, 10" bolt instead of the 9/16". How important is that 1/16". Also do I need fine thread or is course OK. Thanks for the feedback.