I was able to locate a clutch alignment tool for mine when I replaced the ring gear. Call your various transmission parts specialists. I was able to get one for $3. Everything seems to be going smoothly, in fact my clutch was making some kind of weird noise before I removed it and put it back in, and now that's gone, so...

Although I admit I didn't know anything about these witness marks you're talking about, so hopefully that won't come back to haunt me.

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'78 240D