I think 16 gauge should be adequate. It's thin enough to work with manufacturing-wise, won't warp at curing temps for my purposes, and shouldn't flex too badly even in high winds if the side supports are well constructed and properly welded.
For what it's worth, using a tig would be preferable over mig welding if they're going to be powder coated by any other shops. Migs, especially using flux core, introduce a lot of air that can blow out ("outgas") when the parts get hot. I take steps to prevent it but not everybody in my industry does.
If it's a simple matter of a couple clicks, can we see what these same cars would look like with body color louvers? And maybe even white to tie in / set off the side stripes and a chrome look (I know, tough to create in photoshop).