Obviously as it's for an XC bike I only need a top guide.
In fact, BIG LOL's :lol:
One person's XC, is another person's freeride, is another person's canal path pootling. If by XC you mean riding offroad over fairly smooth terrain reasonably quickly, but mincing down anything steep, technical, rocky or bumpy, then you'll be fine with a top guide only. Or if you mean by XC that actually you're a featherweight XC racer, and you're exceptionally smooth as well as quick, you'll probably also get on fine.
If by XC you mean like I do, "ride everything but I'll stop and wait at the top for a minute to catch my breath back so I can really enjoy the steep/rocky/technical/fast downs", then you'll be wanting a proper chain device with a lower guide too.
Can't stress just how happy a good chainguide makes me feel, knowing my chain just isn't going to come off ever, and at the same time just how stressful a sub standard chain device is when you're constantly worried about the chain coming off and jamming, and having to fix it etc.
FWIW I ran a Superstar XCR for a bit. Wouldn't go back in a month of Sundays, not if I was paid to. Now have an MRP Lopes, only a fraction heavier, but a whole lot more expensive, and well... It just does exactly what it's supposed to, all the time, no fuss. With the Superstar I was dropping my chain at least once or twice every ride, often the chain jamming so badly I'd have to split it to get it out again. Fitted the MRP and not dropped the chain once!
Oh, and 38T does sound a little tall. I'd suggest trying maybe a 34T to start, then going either way as you see fit. And should be no chainline issues if you mount it where the middle ring used to be on the cranks, as this is the optimal position anyway.