440 m code block

i have an M Code Cuda fastback that i am putting together this winter. can someone answer a simple question for me. i have read numerous original 1969 published car mag articles that state that the M-Code Cudas were factory 383 cars that were sent to Hurst for them to convert to 440 engine cars. but i have also heard guys state that ALL the M-Code Cudas were built by Plymouth on special assembly lines at Chrysler plants. does anyone have the "final" answer to this question with some reasonable evidence to support your view?
Hurst was not involved with building any of the 1969 M-code A-bodies. They were all built by Chrysler Corporation.