The 1980 model year was the first year for 3-way cats & O2 sensor systems. The engine was de-rated to 160 HP vs. 180 for earlier ones. This was due to lower compression ratio & lower overlap on the cam timing. Im not sure if the lower compression was due to modifications to the pistons (only) or also an alteration to the combustion chambers in the head castings as well. So
the heads may well indeed be different check with MBZ to see if the part numbers match between a 79 & 80.