hm... don't some of the 70's SOHC xs400's have 'squared off' fins on the jugs and head as well? Say, like this 1977?
At least the website I found it on using google says it's a 1977. His engine (1980 SG) is way more rounded than that, but otherwise appears the same.
Here is a picture of the engine in question. The valve cover doesn't look like there are two cams in there to me, but what do I know.
If it does happen to be a DOHC engine, would that be an amount of work and parts to swap in? I see it does have a mechanical tach, whatever it is.