You might be pleasantly surprised, spacers on the BB might not even be needed. but as tommy says only use them for minor adjustment. I'd say that you'd probably want to mount the chainring you want to use to the outermost mounting on the crank, I'd speculate that that would give you the best chance of lining it up with the rear hub. Be careful when choosing the chainring, i'd recommend smaller rather than bigger, and remember to compensate with the rear cog. you dont want to start fixed-gearing with a high ratio, or you may end up doing what i did and knock yourself unconscious! 42 x 16 or 48 x 18 are supposedly good ratios, I'm on 48 x 16 at the moment and i dnt like it... scary going down hills without the leg muscles to slow yourself down properly :S
You might be pleasantly surprised, spacers on the BB might not even be needed. but as tommy says only use them for minor adjustment. I'd say that you'd probably want to mount the chainring you want to use to the outermost mounting on the crank, I'd speculate that that would give you the best chance of lining it up with the rear hub. Be careful when choosing the chainring, i'd recommend smaller rather than bigger, and remember to compensate with the rear cog. you dont want to start fixed-gearing with a high ratio, or you may end up doing what i did and knock yourself unconscious! 42 x 16 or 48 x 18 are supposedly good ratios, I'm on 48 x 16 at the moment and i dnt like it... scary going down hills without the leg muscles to slow yourself down properly :S