I doubt the schwinnbikeforum will be of much help because:
* the FFS system is a Shimano thing - and very few people know anything about it.
* the bike was made by Giant in Taiwan for Schwinn, not by Schwinn
The bike you have already has a quasi fixed gear hub; the gear cluster on the rear hub is not a freewheel, but threads on to the hub just like a freewheel. You need a TL-20 tool to remove it, underneath you will find the standard threads for installing a freewheel.
At the front, you can leave the FFS system in place, of remove it entirely. To remove it, you will need a standard crank puller to remove the left side crank. Next there is a jam nut and lock nut on the left side; remove these, and everything falls apart - pull the BB axle, chainrings and right side crank out from the right side. There are loose balls. The two cups thread into the frame, you'll need the TL-20 to remove them from the frame; the BB bracket is threaded ISO/English standard; the two cups have opposite handed threads. Once you have removed the cups from the frame, you can install any standard 68mm BB, like a Shimano UN55 for example. Along with the crankset of your choice.
Many World Tourists; especially the later ones like yours, had steel rims - these are heavy, slow and provide poor braking. Strongly recommend replacing the wheels entirely with modern aluminum alloy rims. 130mm OLD will squeeze into the frame with only a little pulling.
I'd go with ISO622 (700c) wheels.
The original brakes SUCK - get some Tektro R556 or R559 - the "nutted" type, not recessed nut. The will work fine with the original brake levers.
The original fenders are steel, and quite heavy.
Some were equipped with awful steel handle bars - if you have steel, replace with aluminum alloy bars.
See also:
http://forums.bikeride.com/thread-3216.html