Hey Chevyrestoguy. The firewall doner that I have has a few cages that I will be using. My cab is also missing them. In fact, I had to weld the a hex nut that was used (instead of the original cage and nut) to the cab to get the bolt off. It was a rusted mess. I just got lucky that I was able to get this piece of a truck. Can I send you pictures and measurements to help you recreate one?
If you want to see a unbelievable protouring truck that does beat the Camaro's and Corvettes, check out this build. This guy went over the top.
http://hrjinc.com/projects_shop_57_truck.php