What to do if it rains?

The rain isn't the problem, it's the water hanging around afterward. The parts that rust aren't just parts that get wet, it's where dirt and junk collect and then get wet and stay that way (lower quarters, fender dog legs etc), or where water can puddle and collect and hang out (floor boards, trunk well, etc). Water by itself just runs out or dries up in most climates.

On a fresh rebuild you shouldn't have any dirt and road grime packed into the usual spots, and the seals and gaskets should keep it out of the trunk and interior.

In other words, not a problem. It's a car, it's meant to be driven.