You don't have many options for what you want to do. Microsoft does not provide vertically elongated ads, so to do what you want, you would be displaying ads where the text is at a 90 degree angle. I'm not aware that this is against pubCenter rules, but it makes the ads generally unreadable - so you're unlikely to get users tapping them except by accident (which works against your eCPM).
If you set up the game as LandscapeRight/Left, the ad-sdk detects the changes and rotates your ads accordingly. That said, there is one way that I know of to do this, and it's a bit of a kludge. Even though your app is in landscape, you would set it up as portrait-only. This will make the pubCenter ad be vertically elongated when you view the screen in a landscape orientation (because no landscape-orientation-change event would fire). Then you just pass a rotation angle or transform to your Draw and DrawString calls to compensate for the 90 degree difference.
Again though, I don't advise doing that. Also, Microsoft ads can forced into different sized rectangles, but they only come in two native sizes: 480x80 and 300x50. 300x50 might be a better choice for landscape ads during gameplay, although still not ideal. Shrinking either of these ads to a 216x70 rectangle will affect their readability - and may be against pubCenter/Marketplace rules (I'm not sure). You could also also decide to only show ads during level-changes and other non-gameplay screens although that will limit your impressions. (or you could show 480x80 ads on non-gameplay screens and 300x50 ads on the gameplay screen)
Good luck, Dan