There were different pistons in some MOPAR type AFBs, but the standard piston is the same. For three step the Primary jets are taller, the rod is longer, and you use the cap with the protrusion. Spring rate was the same for most mopars. So that allows the rod to operate over a longer range and use the intermediate step.
If you're going to two step use the early standard jet (shorter), the 2 step rod and the flat cover.