I agree with HillRider. There's nothing unusual about the FFS bottom bracket shell. All of the freewheeling stuff was mounted on the crank.

I was a Schwinn mechanic when the FFS stuff came out. Schwinn made their own adaptations for their version that fit on the one-piece steel cranks, but everything else was the same. The "freewheel" was set up so the cogs would rotate if something got caught in the chain (a safety feature), but the main freewheeling mechanism was (duh!) at the center of the chainwheels.

What you probably have is the version that fits a threaded bottom bracket. It's pretty weird- don't be surprised if today's average bike mechanic can't figure it out. The cups thread into the BB shell before the spindle is inserted, with threaded cones riding on the spindle. The cranks aren't square-taper, they're Octa Joint- the '80's version of Octalink. The disassembly will be in this order: cranks, chainrings (I think there's a snap ring), left side bearing cone, spindle, BB cups. I can't remember which tool removes the cups, but it may be a conventional hook spanner.

If you wanted to reuse the rear wheel, bear in mind that the FFS freewheel uses the older style Shimano freewheel tool: http://biketoolsetc.com/index.cgi?d=...em_id=BR-CT6MB . The newer one doesn't fit.

There's a page about the FFS in the 1982 Shimano catalog on the Harris Cyclery site:
http://www.sheldonbrown.com/shimano1982/pages/35.html

If it's going to be a single-speed, you're going to have to get used to pedaling without switching gears! (I have a single-speed Schwinn LeTour. It's a dandy kick-around bike.)