Between the leafs? I wouldn't worry about that for a split second. If you actually drive your car you won't be able to maintain a perfect paint job on your leaf springs.
As far as pinch welds etc, the undercoating suggestion is probably the best idea. Although if you drive your car at all, you'll probably end up with some surface rust here and there on the undercarriage. Some surface rust is no big deal, the biggest thing you want to prevent is trapping any dirt or crud in the pinch welds, as that will hold moisture and you'll actually get rust through.
From the factory the bottoms of these cars weren't even painted, just some random overspray unless it was a factory undercoating car. Some rust staining is perfectly normal and won't hurt anything. Joints and seams would be the only thing I would concerned with, and only if they were actually trapping dirt or water.
If you actually drive your car, it won't be perfect. You'll get grease and oil in the engine compartment, rock chips on your fenders, and dirt everywhere. But I'd rather see a car at a show with a little bit of dirt, oil and rust than some perfect, overly restored garage queen.
