CMJ came back as 3 possibilities:
CMJ 1974 350 160 HP 4-Barrel t400 A-Body, police
CMJ 1975 350 145 HP 4-Barrel THM 350/400, A, B-Body, federal
CMJ 1980 305 - automatic trans
In this case, you need to find the Date Code that is on the back of the Block. Not real sure on the newer engines, but the older ones have what looks like a Slotted Screw Head, then a Letter for the Month, then some Numbers for the Day and Year, then another Slotted Screw Head.
Although, if you research the Casting Number on the Block, that could possibly narrow it down some more, if getting to the Date Code is not possible.
-Parting Out over 75 '58 to '73 Full Size Chevy Cars-
-National Impala Association
-Vintage Chevrolet Club of America