Excuse me, yes, 71 blocks not 81 like I wrote. If I had to guess, the motors are out of vehicles later than 71. If I'm correct, Ford designed castings and stamped them with the year they were designed, not necessarily the years they were used. For example, the E7 head was designed in 1987 but used on a lot of different years up to the 90s.
My guess is that the blocks were designed in 71 and used throughout the following years. The heads were designed in 73 and used up until the early 80s. Your motor could be out of a lot of different "Model years" as far as the vehicle go, but the motor would be the same.
Like, one motor could be out of a 75 Lincoln but still have a 71 block with 73 heads. At least thats the way I understand it! Are these original motors that have never been taken apart?