Lets try it this way with the dates of your own examples for the 'documentation'..........

FACTS;

1. The original Mylar door stickers had to show the actual build month & year per Federal Law. So the date shown indicates the FINAL assembly date when the fully completed car was physically ready to drive out of the plant.

2. As such, the Mylar door sticker was printed & applied right before each car left the plant.

So with those two facts in mind, if the Mylar door stickers on two of the three examples are actual originals showing March 1970, then it is impossible that they left the plant with engines cast in April.

So here is an elective homework assignment for you, note each of these things;

Date on the original Mylar.

Casting date on the engine.

Final assembly date on the engine.

If those three dates are original to any of the examples, neither of the last two dates will predate the date on the Mylar.

Hopefully that helps explain it?