Today, I had the Polara rustproofed by Krown in Ottawa. The guy who did it owns owns a 1967 Crysler Newport himself and he said with a car like this, it's best to not drill any holes and because much of the car is so accessible and he knows the ins and outs of it, he did (and wanted to do) a real good job. So, that's step 1 out of the way.
As for spring/summer/fall shelter, I'm going to be doing what articsteve suggested by laying a good base of vapour barrier layers and packed aggregate. I'll also be buying a portable shelter to keep it out of the elements. With the rustproofing and this kind of shelter, the car should stay good and dry, not exposed to UV to keep the original paint good. She should be good for many years to come!
Thanks to the advice of forum members!