lippy,
Good advice as usual from Bob and fellow CC members.
I used PEM nuts and allen head / button head stainless button screws for my transmission tunnel, it turned out great.
Below is link to a web page with a chart, they come in #4, #6, #8 and #10. The chart has recommended sheet thickness and hole size needed.
PEM Self-Clinching Nuts, Type SP - Unified On S.W. Anderson
If you want a Bay Area over the counter source for PEM nuts and the button head screws, along with just about any other fastener you can imagine, try Olander in Sunnyvale they have a great selection.
THE OLANDER COMPANY INC.
144 Commercial Street
Sunnyvale, CA 94086-5298
Telephone :
800 538-1500 or 408 735-1850