Compression check is best, not easy so best to combine w/ injector removal & test. At a minimum, check ring blow-by at idle (many youtube).

Don't know of alternatives to the M-B tranny. You can bolt up a later 700.4xx tranny (86+ cars?) since 85 CA uses that.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans