Firstly, thanks for the quick and fairly detailed response. I was under the impression that all 300SDL's are the 6cyl?
I know I haven't ever overheated it [had it since 140,000]-- It's leaking coolant, is why it was taken in, in the first place. I feared some sort of head problem. It just started [noticably] "misplacing" coolant about 1-2k ago. The coolant light would come on going up a hill, then quickly go out as the car leveled off. I added some coolant, then, about a thousand miles later the light started coming on again. So here I am. Is it possible that the previous owner overheated it and the crack has just now become very apparent? Lastly, the question I'm not sure I want answered. How much does a new head cost?