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.
Well, I am proud to say that I drove my MKII in the rain last weekend when I caught up with a cold front on I-10 near Columbus, Texas. The car did surprisingly well, and the view over the hood with the windshield wiper going and the rain drops moving up the front clamshell was all very "LeMans" like. I found myself humming the theme from "A Man and a Woman"!
These cars will rust just as fast as the cars built back in the 60's, how's that for originality? Hi, I did extensive weather "proofing" of my car and I'm glad I did. Did it make it water tight, not possible! On my first road trip my right shoe filled with water from the ducts in the front clip (heater, etc.). I posted much of what I did so if you're interested do a search on my name. BTW I've driven my car more than most 25K+ miles, maybe more the speedo stopped working around mile 2500. Needless to say I love my car and wouldn't have it any other way besides who needs to know how fast you're going. Having my hair on fire is enough for me.