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?