You can see the back side of one in this pic. they go thru the metal frame of the dash, when you put a screw in them they expand like any screw anchor which seems to make them hold tight well enough.

I seem to recall having to have to go to AMES GTO catalog to find those pieces. For some reason no one lists them as a Firebird part.
I think this is the right one. Part # A259W
https://secure.amesperf.com/qilan/Detail_Web?part_num=A259W&order_number_e=Mzc4NzAwMg%3D%3D%0A&web_access=Y