Some of your questions don't make sense. No one actually knows what the factory carpet weighs, no one is going to protest things that there is no answer to.
The heater box only weighs about 10 lbs, so it shouldnt be your major decider. The cars came with enough gear options so that you will not have to worry about that issue.
The rule book is pretty clear that superseded parts are ok, so there is no reason to worry about things like specific part numbers. You will be fine if you have a few Z28 basics, like an intake manifold, and possibly some double hump heads, but there is no way to prove performance improvements or not from other parts. Bare with me purists, but at its core, a z28 is just an engine option. Especially given the legal modifications you can do to the suspension. Put a z28 intake on top of a 4' bore 302 engine, and back it up with a 4 speed muncie, and the rest for your purposes is just another restored camaro. Don't forget some emblems.