Ok, what I know of the MPC Mustang II's:
1974 was a new tool
1974 the Firefighter and Dyno Don prostock released
1975, body retooled to a 75 (hood and header panel was different)
1976 the kit '75 was reboxed as a 76
1976 the Cobra II was released
1977 the previous kit was reboxed
1978 the King Cobra version was released
1978, 80, 83, 85 and 2005 the Pro Stock kits were released with different boxes.
ALL pro stock versions are 1974 bodies.
Now, an interesting observation from the AMT/Ertl Street Custom Mustang II was AMT in the box but boxart was an MPC. Suspension shown on the side panel was an MPC kit.
So is the MPC variant still around? Hope so