There are various options out there.
KBS rust seal.
Cavity Waxes etc..
In many cases these are applied by a using a tool that allows you to paint in hard to reach areas.
Eastwood's offers kits like these:http://search.eastwo...t Proofing Wand
Where you get a wand to spray into hard to reach areas etc..
I've seen some examples where holes are drilled into sill panels, rockers etc.. So that a spray can be applied, obviously I'd rather not drill unnecessary holes if not required.
My approach on my car will be to rust proof areas I have access to when doing rust repairs and make sure surfaces like inside of panels that were not adequately prepped at the factory back in the day have etch primer with some kind of rust proof paint over the top.
Once the car is fully painted I'll look at wax coatings applied to common problem areas for S30z's, but I plan to keep my cars garaged anyway (so many benefits to this I won't even start).
I would honestly not plan to keep a 45 year old S30z out in the elements if you can avoid it for too long, even if it means renting a garage to store it in. In Australia you also have to worry about hail damage which is almost impossible to predict..