Not sure about the chassis difference..for the duc 749 and 99 you are absolutely right-they have same chassis/bodywork but different engines..as far for the jap 600 not really sure.But if there is a difference it will be very very very small..look at some pics of "naked" cbr 600 VS "naked" cbr 1000 and you´ll see what I mean.Cheerz! 
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