The MGF spring rates will be too high for the K3 and most likely to result in very stiff suspension and a 4x4 style ride height due to the K3 being much lighter overall.
I am facing the same issue by fitting MGTF front and rear kit and know that I will need to downrate and have lower springs made to suit, but as yet do not know what they will be.
Mike Satur does a kit at over a grand that may be adjustable, but is a bit more than just replacing the spheres with spring boxes
http://www.mikesatur.co.uk/index.php?ro ... uct_id=833J