1. Adjustable cam gears can be used to compensate for timing changes due to decked blocks / shaved heads. Even without a decked block or shaved heads, if you have upgraded cams (JWT 400/400+/500), you are going to want the cam gears to maximize performance. 2. Use of a multi layer metal head gasket requires the block and cylinder head surfaces to be absolutely perfect. Any slight imperfection and you will have a leak. OEM head gaskets are much more forgiving.