Interesting you should bring this up, as I have been discussing the same thing with someone. US rules are clear when it comes to maximum allowable offset:
FXS, Section 9: " 39" minimum wheelbase. 42" Maximum width. No more than 4" of total offset. Offsetting of axles, center of frame to outside of right tire is to be 23", and from center of frame to outside of left tire is to be 19" giving a total offset of 4". "
That being said, the debate is this: FX/S & FX/T with no offset would be 38", but a prepared chassis is allowed 40" maximum....sounds like a call to Manny is in order?