You only need to replace the ring gear. It needs to be removed and heated to expand it and replaced. I did mine a fer years ago. Its a 3 hour job. Also inspect the damper plate while it is apart if is original it wont hurt to change it. The gear is a 157 tooth for the 12 inch flywheel
Cost $25.00 for ring gear and $30.00 for the local machine shop to remove and replace.