I saw straight 6 and immediately thought of the old aq170s. I didn't even read where you said they were 165's, and now that I look again, I should have known better. I'm pretty sure somebody cobbled those together. Volvo didn't start using GM blocks until about the late 80's, and by that time, the straight 6 was long gone. The very last of the I6's from Merc was in 80/81.....
On your first set of pictures. The one on the right. That has what looks to e a Volvo number on the valve cover. Whats that number say?