Replacing switch box and cleaning connections

[brooksville_rebel]

After testing (by mechanic) seems that my switch box went out on my 1987 Mercury 70HP 3 cyl. The nuts are somewhat croded greenish. So figure to either replace them or clean them. I was wondering if I should get SS ones or brass or does it matter ? Or just clean these ? If so whats the best way to clean them. I recall reading one time that they shouldn't be sanded cuz that they normally have a coating but considering they are green figure its gone so doubt that matters, but just want to make sure I get good connections putting on the new one. After getting on should I spray with WD-40 or like crc or some other spray to help protect ?
Thanks

 

[brooksville_rebel]

No Title

I went ahead and lightly sanded and sprayed with CRC 2-26. Anyways a different concern now. The new switch box has what I assume is a ground wire coming out of it and the old one did not have this one. I have attached pics of them.
Do I wire up the black to the ground some were on the engine block or tuck it away ? I could not find a answer searching. I don't want to pop or fry anything.

Thanks