I would still recommend you fuel-proof the firewall. You can use either clear, or colored dope, fiberglass resin, or epoxy. If you go with epoxy, there a a few ways it can be applied. First you want to use a 30 min, or longer, type of epoxy. Fninshing epoxy works great because it's so thin and doesn't set-up quickly. If you use any other type, it can be thinned with alcohol, or better yet, place a fair amount on the surface and add heat from a heat gun to thin it out and it'll also wick into the wood and joints better.

Any of these metods will help protect the wood from fuel soaking which leads to weak wood and joints.