Re post #2, I put an earlier engine (1982?) in my 1985 CA 300D, so seems the engine mounting points are identical since I used the brackets already on the earlier engine. The 1985 CA car does use the later .422 transmission, but the engine bolted up fine. If anything wasn't optimal, I haven't noticed in ~30K miles of driving.

I doubt the seller could "feel" low compression, but you can. If you turn it over by hand, you can probably tell. You should feel each cylinder strongly fight you - 5 in 2 crank revs (mark w/ chalk) and you should hear them slowly hiss down, >5 sec meaning "good compression" to me. Once it hisses down, you can continue turning. I turn my 1984 & 85 engines over using the bolt on the pwr steering pump pulley, since much easier than accessing the crank bolt. People say not to do that, but I don't know the concern. If the starter cranks it over fast enough, you might be able to tell excessive "blow-by" from puffing out the disconnected PCV hole. One likely possibility is that air got in the injection system, either from running out of fuel or a cracked rubber hose. That is easy to fix by cracking the nut (17 mm?) on top of each injector and cranking until you get fuel dribbling out. Tighten it as the fuel is flowing.

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