Jch,
Dave is right... best to use an aluminum plug. When dissimilar metals and moisture are combined it can be a recipe for galvanic corrosion. Using a good thread sealant can mitigate the potential problem by keeping moisture from entering the threads, where the metals are in intimate contact. Consider this also... I think builders switching from the old plastic fuel header tanks to the aluminum ones often transfer the original brass fittings. It's probably not a huge issue because again, the threads should be well sealed and they're not constantly exposed to water. I used all aluminum fittings on the header tank. You can get an aluminum plug from Kitfox, I'm sure, or from any of the aircraft part suppliers, automotive "speed" shops, etc.