Don't really know what you're asking Dave. Taking pics of the moon with your lens racked out to 400, shouldn't be a problem. Since the moon is giving off so much light, stick it on manual, ISO 200 with the lowest F number your lens will allow at that range, then set the shutter speed at 1000, shoot it and check the results. If its too dark, then gradually reduce the shutter speed a bit at a time until you get the desired effect. I'd have thought at such a fast shutter speed you wouldn't need a tripod, but if it is still blurred, use a tripod and shoot on timer (this is because the camera will move slightly when press the shutter button). Also, at 400mm your camera will get in tighter with the smaller sensor (1.6 x 400).
Oh, I read your post again... are you having trouble auto-focussing? Switch the lens to manual as well and manually focus. Defo best results with a tripod.