I am setting up my mbx-5 as an on road and have a couple of questions

[SpEEdyBL]

Are you racing? Just go with stiff springs like 6+ lb/in front and rear would be ideal with at least 60wt shock oil. Center diff fluid should be as high as possible if you are aiming for speed runs or at least 30K wt in general. Whether you change the center diff fluid or not should depend on how stable your car is or how stable you want it to be.