I'm guessing not many people have tried it. Get the dimensions of the rim. check to see if it fits between the forks. Not sure how it will go with the speedo, either. Or how well the 250's brakes will connect to the larger discs of the 600.
i know a guy with an 600 but its in the shop so i'll beg him to loan me his wheel just to try i hope he says yes. i'll get back to you with the results.
No need to borrow the wheel... just get him to run a tape measure between the wheel and measure the bearing to bearing distance. I'm thinking your chances aren't very good however as the FZR400 has larger tyre's than the FZR250, so the 600 is likely to have slightly larger tyres (and therefore rims again).
i have bought the bike off shandu and i got the 600 front rim to work but there is just one problem the disc on the rim are bigger so the calipers cant fit but the both bolt on the same as those on the original rim so am gonna take off the disc on the 250 rim and change them with the 600 and see if it will work
