First you have to do slope stability analysis without the pile. You should be able to get a range of possible slip surfaces from this anaysis. Design the pile length such that the pile intersects all surface plus some allowance for extra embedment. Spacing will depend on size of pile and shear resistance needed for an acceptable factor of safety.
Use the Viggiani analysis (Napoli University) based on his 1981 paper. Deduce the FoS at existing conditions, calculate the force/m run that you require in order to enhnace the FoS to the levels you set and then use the Viggiani method implementing Broms ultimate lateral resistances and shear forces in order to calculate your piling schedule.