I have a 2002 kawasaki prairie 650 and one of the back seals is 40X52X7 .. my question is , would a 40X52X6 work in its place ?

 

Yes, 1mm on width is not that important, particularly if it is narrower, but ensure the lip(s) are the same, i.e, if the old one has one lip with a spring ring and another without, the new seal will need the same, or two spring ring lips.