It depends on what kind of tools you are using to play nds games, for example, flashcard or TWM++. But I think this great post may solve your problem since you don't want to strech the video:
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Great, I can't post links because apparently I just registered this account. You'll have to search "Widescreen cheats for DS games on 3DS" for yourself. I'll head back and edit this post when I CAN post a link.