What the heck?
Narrowing the differential and tubing wheel wells has nothing to do with what suspension type you use, And No, you can not and should not try to use leaf springs with a four link suspension. Why would one want to keep leaf springs on any suspension type unless required by rules in a specific stock racing class?
Two Coil over shocks with springs will weigh about as much as the bolts and hardware that holds just one leaf spring on your car now, not to mention the weight of the leaf spring itself X 2...............
Bruce