I have a couple questions for you before I can give you a solid answer...
1. Why are you dead set on S30 or Z31? both being independent rear suspension which isn't preferable at a dragstrip and you said thats what you are most concerned with...
2. Why not go for a car thats already small block ready and better suited for what you want to do (drag race)?
3. You haven't even bought a car yet, and you are already planning to build the stock engine till it blows? then swap to LS, why not go straight to the finish line?