Sweet B6!
The three biggest factors in making a bike ride nicely are seat height, gearing, and tires.
On many of my vintage cruisers I use a taller Sting-Ray seat post to get the seat up to a reasonable height, makes a big difference being able to use more leg muscle.
Any Schwinn with a big ring up front requires (at least in my mind) a 22 tooth cog on the back in order to get the gearing down to something my knees can handle. And let's face it, there's a reason they stopped using the 52 tooth sprockets in favor of the 46 tooth, heavy bikes benefit from lower gearing.
Tires and tire pressure are the other biggies. Modern tires, even vintage looking one, are often much lighter than the originals. You'd be surprised how much better a 45 pound bike will ride when you take 12 ounces off of each tire. Modern tires will also air up a bit higher, and the difference between running 25psi and 40psi is significant.
Cheers, Geoff