I've never done a swap of non-original engine or tranny into anything -- I've heard you have to be well tooled up to pull it off. But from what I can gather, I'm under the impression that it's a lot easier to put in a stock engine/tranny combo -- IOW, don't just put in a new engine, put its stock tranny in with it. Much easier to have a driveline adapted or remade with the back half OEM and the front half stock to your new tranny, than to adapt a bell housing.

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1986 300SDL, 362K
1984 300D, 138K