First off Nismo clocks are like rocking horse sh!t so bear this in mind! and if you dont want them I will gladly take them off your hands.
Right you can simply plug and play the clock although if your JDM has been converted the speedo will now not read right again. (not sure how to adjust this sure someone else can shed some light)
Secondly the millage is stored on the cluster therefore it will change the mileage according to the last miles on the speedo.
I have got round this by fitting a MPH facia to a JDM cluster as the position are the same.... this retains correct mileage, and all other settings.
Hope this is helpful .... PM if you want anymore info on this