i have a 78 camaro and im replacing the lower cowl but before i do i would like to coat the inside of the fire wall in something since im never going to get back there to it again. what should i coat it with?

Dare I say it....POR-15 or your other favorite rust converter. They are all formulated about the same, and will protect inaccessable areas such as that. Just wire-brush or sand the loose rust off as good as you can. Then brush the converter on. Use a broomhandle taped to the brush, if necessary for hard to reach areas.

I would coat it with black epoxy primer. Two coats and you can brush it on if you want to.
Epoxy will last almost forever in areas not exposed to direct sunlight.

There are horror stories connected to POR-15, not so with a quality epoxy primer.