There is no such thing as "rust proof" when talking about mild steel. The SPF chassis is stamped and assembled from coated steel that is rust resistant. It can rust, particularly where multiple layers of material come together as the welding process disrupts the coating.
Realistically with the type of use these cars get the chassis will outlast all of us posting here. The FAV cars, as they became older and having no corrosion proofing past primmer paint and being used as daily drivers, in some cases were exposed to conditions that most new GT40s will not be subject to. The photo of the ice encrusted MK I on it's delivery drive circa 1966 is proof of the type of lives the FAV cars often lived. I have customers who have never gotten their Superformance GT40 wet!
If corrosion is of concern you can easily treat the chassis with products to reduce (you will never eliminate) rust. Thin wax type sprays can be applied at seams, in the sills and other areas without making a gooey, sticky mess.