Yeah, something went through the oil. My guess is whatever broke out of the intake bolt threads on those 906 heads probably didn't get cleaned out and happened after the build at some point.
You're right to be concerned and taking it to the machine shop. Too bad you weren't in Washington I would have liked to have helped you out on that stuff.
For putting money into an engine that's going to be up to you, does it look like the heads will work on the 383, will the valves clear the bores ?