Setting Up 640 x 480 Footage in FCP to Output at 16:9 with Letterbox on Sides

Hello - after searching online for an hour and not finding an answer, I need to ask this question:

I've created a FCP project with a variety of clips from old home movie footage, which was imported into FCP. When setting up this project, I chose the option where FCP creates the project properties based on the 1st video clip ...

And this yielded a project resolution of 640 x 480 at a rate of 29.97p. The result is that the workspace shows the video clips at their original aspect ratio, so the height fills the entire vertical frame and there are black bars on the left and right sides to fill in the rest of the horizontal 16:9 workspace.

When finished, I exported to YouTube using the "Large 540p (640 x 480)" option. I was hoping that when viewing this on YouTube, the 16:9 ratio with left and right black bars would be retained and the resulting video would appear like this: https://www.youtube.com/watch?v=_LKSW_RK8Ss

Instead, it ends up looking like this: https://www.youtube.com/watch?v=9uBoloMD18c

I'm trying to achieve a result like the Noel Gallagher video linked above, but I'm not sure how to do this and am not finding instructions anywhere for this use case...

One related question: at https://support.google.com/youtube/answer/6375112 it says "For best results, avoid adding padding or black bars directly to your video. This interferes with YouTube’s ability to adjust the player dynamically to your video size and the viewer’s device."

This would make me think I should not do what I'm trying to do! However, since the (very recent) Noel Gallagher video *is* doing this, I figure if they're doing it then it must be the correct thing for this situation...

I'm hoping to publish this soon, and will be thankful for any assistance and advice on how to proceed!

- Jim