Thanks to everyone for adding useful info. I'll add that since this post, I found that the unused "large round plastic housing w/ cable" on the firewall of my 1985 went to the "rack position sensor" on the injection pump. My replacement engine & IP was newer and doesn't have that component.

Another tip I learned since. If your T-stat housing has never been removed, now might be a good time to remove from the head and change the large "bypass hose" on the bottom. I used blue silicone hose on both 300D's (ebay, vaguely recall 42 mm ID, but measure don't flame me). While housing is off, a good time to unbolt the timing chain tensioner since most internal springs have worn flat on one side (Peach Parts ~$7). Of course, good time to replace the upper timing guide rail, but always pick your battles carefully and allow a large block of time.

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