You are right the 340 never changed (assuming X and J are relatively the same) their cylinder heads between pre-emission, or post emissions.but they did knock down the compression height with different pistons in the emissions era 340's '72/'73.
The lower compression knocked down the NOx. After that, they incorporated a egr system to knock down the carbon monoxide and hydrocarbons. Then they went to closed chambers and small valve to increase swirl in an attempt to increase burn efficiency with a leaner mixture, and lastly changing the magnum block to a lower deck height to optimize quench.