Hot Rods Running Boards & Splash Aprons

Well fellow HAMBers! Many thanks for the good advice you have already shared to help me build my first street rod. We are closing in on it but now we could benefit from your advice some more. The pic shows our concept for the running boards and splash apron. We plan to run the exhaust between the apron and the frame and then exit via oval outlets just in from of our 20" wide Hoosiers.

But now the question. What material and gauge should we use for the running boards and apron? Is regular mild steel sufficient or do I need to find a source for stronger material. Is corrosion resistant material available? Necessary? Thanks in advance for any assistance you are willing to provide. Appreciatively, Michael