This is a question that comes up frequently;
Mopar did not intend to offer a 340 engine in a Challenger in August 1969 when 1970 Challenger production began. As Matt denoted, when Dodge saw that the 340 would be available in the Cuda, they decided to create the A66 package which was not made available until September,1969. Early cars had the 340 decal hood callouts because they did not originally plan to offer the 340, no metal callouts were yet produced. In our A66 Registry, the earliest SPD for A66 Challengers are 9/29 for H0E cars and 9/22 for H0B Challengers. There are many October 1969 A66 builds for both factories, once the public became aware of availability. As per Hamtramck Historical, a Mopar bulletin dated 7/23/69 indicated that the A66 Package would be available in early September. The package was changed as of 10/20 making F70-14 tires standard instead of the 15 in wheels.
Regarding TA Challengers, it has been mentioned that the future Trans Am Challenger Program was the impetus for adding the 340 to the Challenger. There was a Mopar bulletin dated February 20, 1970 released to dealerships introducing the A53 TA package. As to how early Ma Mopar was envisioning the T/A package, hard to say. They kept the production dates for the TA to a tight time frame in the spring of 1970. For anyone interested here is a Hot Rod article on the TA340 cars.
https://www.motortrend.com/.../fast-1970-dodge.../
