6 pack Challenger question

[69hurstSC]

just got done looking at a factory 318 70 Challenger. car now has a date coded 7 21 69 440-6 in it. intake is from 6-14-69. as a rule of thumb its usually said the motor and body shouldnt be more than a month apart. w/o a body i'm not sure what this motor is out of? the current owner says its out of another 70 Challenger, but were they building them as early as July/August?
Anyways, the car needs a lot, so my intentions would be to pair up the Challenger with a 318 and sit on the 6pack. so can anyone help me on when they started building Challengers and what else can I look for to verify what the motor came out of?