You could get yourself some 590s, aka EA3, aka 650A, aka 26x 1 3/8" (Which is what your s6 rims purport to be, but they are 597s instead of 590s.) The bonus of going with this size isn't just (slightly) better selection of modern rims and tires, but you also wouldn't have to change your brakes due to "reach" issues.
A Collegiate, though, has a lot of room. You could run regular 559mm 26" wheels with tires up to 2" thick in the back; measure your fork clearance, but it should be able to do the same. This would require much longer reach brakes, and drop your ride height down considerably. You could run 650B (584mm) rims/tires, but you'd likely need different brakes with a slightly longer reach... or you could run 622 (aka 700c) with shorter-reach brakes.... You have a lot of options; the question, really, is what are you trying to do?
-if you want a faster, more "Roadbike" ride with plentiful options in rims/tires, go 700c. (You'll have room for some medium-thick 700c tires in your frame, so....keep that in mind, too.)
-if you want a plusher, more cruiserish vibe,and nearly limitless rim/tire options, go with 559s aka 26" aka s2....
-if you want to keep your current brakes and keep the same basic ride experience that you have now, but you want to run some aluminum rims (Sun CR18 is a great option) and more/better tires (Schwalbe Delta Cruisers or Schwalbe Marathons are great), go with the 590s.
HTH
-Rob