Is building a stroked 318 better than a stroked 340/360?

All other factors being equal, the 318 based engine will make the least torque, followed by the 360 based engine, followed by the 340 based engine.

But run what you brung.

And make sure to run long rods with custom pistons and/or custom offset crank because if you're not doing that you're leaving "free" power on the table.