I have tried spraying with WD-40 Rust Version for a week, then switched to PB Blaster for a week. Now am trying AeroKroil for a week. Also tried heating them with a Propane torch, but it won’t get them hot enough. Found this post to be helpful: http://www.camaros.net/forums/showthread.php?t=160328
Discovered that Classic Industries carries Firewall Brackets: http://www.classicindustries.com/camaro/parts/14945.html
I have e-mailed them to ask if these contain the cage nut and if so, I won’t have to be so careful removing the firewall nuts as I will be able to replace them!