It is my understanding on the cars with the full 70 dust trail stripe, that the little "x" is supposed to be facing down. Although it is not clearly indicated on the factory instructions. I could see how it would be easy to flip over and no one would ever notice unless you are specifically looking for it. Most of the cars I look at have been repainted and/or stripes added. And I have seen them both ways--even on different parts of the same car. My car had all the "x"s on the bottom except the passenger side front fender, where they were on top. My guess if it is so easy to mess up on a repaint, the guys on the assembly line back in the day probably messed it up sometimes, too. I don't have any factory original paint dust stripe cars around here to check my theory. But my guess is the guys on the assembly line weren't being that picky.

That said, the first section of the stripe attached to the road runner's feet has the "x" on the bottom (as you said) so logic would tell you the rest of the stripe should go on the same way.

On all the full 70 dust trail decal kits I have seen--the background is gold, and the section of dust attached to the road runner's feet is cut off square like it was cut with a scissors. It cannot be used as-is by itself. (Decal #1 above)

All the "no stripe" 70 cars I see, the background of the dust is black (not gold) similar to the 69 decal and the section of dust trail is rounded, not cut off square. (Decal #9 above).

Again, I'm no expert and my car was a repaint, so I'm sure someone more knowledgeable than me will pipe up.