In my limited experience, older road bikes like the ones you tested have narrower bars than newer road bikes. I believe the ideal width would have your hands about shoulder width apart while riding in the drops.

Note too that modern road bikes have better designed brake levers for riding on the "hoods". When riding in this position your hands will be farther apart than when you're riding on the "tops", - which was what you were doing. Most people on newer road bikes ride primarily on the hoods.

I don't know how hard or easy it would be to find wider bars for those Schwinns but there are different widths available. There are even drops bars designed for offroad use which flare out at the bottoms. For the most part though, any drop bar is going to be narrower than a MTB bar and will take some getting used to.