After ridiculous amounts of reading and Googling, I have my answers - the FZR250 swingarm pivot is a massive 200mm wide (almost exactly 7-7/8"). The R6 arm is WAY too frickin' wide; I'd be past the welds of either frame or arm (or both) to get it in there.
The swingarm from an MC21 NSR250, on the other hand, is a mere 205mm at the pivot. All I have to do now is find one...
