S-6 = 597mm diameter. MTB = 559mm. So 38mm smaller diameter, or 19mm smaller radius. That is A LOT. Brakes are not going to reach, you will need to cold set rear and front fork, and probably file front fork dropouts to fit larger diameter axle.
Better bet is to find another Schwinn with S-6 wheels and make a swap.
Another choice is the 26 - 1 3/8 inch wheel found on old department store 10 speeds (590mm). Yes, those rims would be steel. Should be able to get the brakes to reach, and take a more common tire size. May even be able to find a set of them at a co-op, in the form of a cheap bike.
I picked up a coaster brake cruiser with that size wheel recently. And I picked up a 3 speed yesterday.
If you were near me, I have some great options for you with minimal cost.
Last edited by wrk101; 08-30-15 at 07:21 AM.