I'll 5th the suggestion that an air space at the top of the radiator is normal. As example, for a time I would notice the coolant level in the radiator being down ~2" below the cap in our 2002 T&C 3.8L V-6 van, so would top it off. After repeating many times, I noticed the level in the coolant reservoir had increased. I read "air at top is normal", so stopped topping it off. Indeed, most Owner's Manuals today tell you to check the reservoir level (and don't fool w/ radiator cap), which is probably simpler than trying to educate owners on the "why".
If thinking "2002 engine is totally different", my 3.8L has pushrods, 2 valve/cyl, and the rocker arms are shaft-mounted stamped-steel and look almost identical to the rockers in my 1965 383 engine. The only differences are: 2 less cyl, alum head, roller lifters, oil-pump integral w/ timing cover, and modern fuel and ignition controls. All of these can be upgraded on my 1965 engine (exc. #cyl & oil pump).
BTW, I switched to Evans Waterless Coolant in 6 of our 7 vehicles, so no more boil-overs or corrosion worries. Downside is that if you do have a small coolant leak you are motivated to find it ($42/gal).