How thick should firewall material be?

[belair54]

I am going to have to replace the firewall in my 54 Belair due to PO chopping it up and fabbing his own parts out of fiberglass and not done too well. Any suggestions on thickness of material? My dad has access to some sheetmetal runoffs at work, thought all he had at home was 16 ga and thought maybe that was kinda overkill, but wanted to know what the rest of you have been using.