The A518 (AKA: 46RH) in our trucks uses a NON lock-up torque converter. You can't simply replace one converter with the other.
While the A518 can be converted to a lock-up type converter by way of notable internal hardware and hydraulic (Valve-body) changes, it's usually much more cost effective to switch to a later 47RH trans which by default runs the lock-up converter.
Either way, you'll need the 47RH type engine to trans adapter-plate so as to allow more room for the thicker lock-up converter.
Others with first-hand experience with the details of either conversion will most likely post soon.