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The info you're after for the motor would be for the 440 heavy truck engine referred to as a 440-3.
1978 was the last year the 440 was produced but there were a large number of Class A chassis distributed ; even some re-serial numbered by Dodge till 1980.
I worked for a Dodge dealer from 75>07 , if you post some questions maybe I can help .
Or , if you post the questions in the " Vintage RVs" forums , there are a number of owners of MHs on the Dodge chassis there who answer questions.