best place for ground
You want to mount it back to the frame if you can. You may have an existing hole to use. Make sure you grind off any paint or rust.
If you don't have any dialectic grease to put on the bolt and terminal, you can just give it a nice coat of paint. The shorter the wire the better.
The manufactures stopped mounting them to the engines long ago, due to rubber engine mounts and the engine naturally twists in the mounts when the gas pedal is pushed. It would sometimes yank the battery out of the holder when a motor mount broke. And the wire would tend to wear faster.
Hope this helps and we'll talk with you later....


