I wouldn't worry about a spacer. "crossing the chain" (using the innermost cogs with outermost chainring, or vice versa) is generally not a good thing - the sharper angle at which chain comes off the teeth, the more it wears the chain and the teeth. By the time you get into that gear, you could instead just shift into your large chainring. So, I wouldn't mess with spacers - it sounds as if your chainline is where it's supposed to be.