You will need to ensure that the springs can not fall out of the cups when the suspension is fully extended. This can be achieved by modifying the rear shocks, or by using Corsa rear shocks. At the front the springs can be wired into the cups or small tabs can be welded to the cups to hold the springs in place.
Don't expect the ride to be great.
I've been using fairly heavily shortend stock Micra springs for a good few years now. They have passed MOT's in all that time and I can still just about walk after getting out of the car after a journey.