You mentioned you're using a bmx freewheel w/spacers, so do you have a bmx hub or is the bmx freewheel screwed onto an older style mountain hub and the spacers are being used to get a good chain line
older mtn hub, tried to move the chainline over. It took a different bb spindle as well, but after awhile, I got it right.
Speaking of bottom brackets, have you found that your ss tends to put extra stress on the bb? I just pulled mine apart the other night and found the reason why mine's been developing extra play- the drive side bearings had been totally mauled and the retainer was in pieces. I'm not overly hard on the bike (its still in "testing and development"), so even a used bb should have lasted longer...