I have at least two different types of screws/washer combinations fastening the top of the instrument panel to the dash. What was the correct/original fastener used?

Here is the kit that Ricks sells.
http://www.rickscamaros.com/cgi-local/SoftCart.exe/cgi-local/smpagegen.exe?U+scstore+pfzf7018ff0eaa0e+-p+-c+scstore.cfg+HW-50

Ground Up would carry it too, I could not find a link on their site

AMK is also an option, they're great to deal with. It looks like they sell the kit for $5 which comes with the screws & spring clips.

http://www.amkproducts.com/default.htm