fcp7 and pixel aspect ratio for DVCPro HD and a complete idiot

Sorry to the veterans for returning to this topic yet again, but this is the brain-killer for me.

I have been shooting with a Panasonic at 720p (DVCPro HD), importing to FCP 7 with the DVCPro HD 720p 30 preset, editing in the same sequence setting, and outputting with the current settings to h.264.

When I few these movs with my ancient QuickTimePro 7, they look fantastic, and I've been sending them hither and yon as needed by colleagues, etc.

Unfortunately, I recently opened one of these movs on a Mac that had QT Player X installed, and found to my discomfort that the video was playing at a 4x3 ratio. Of course, the useless QT Player has no way to set the ratio, so it occurs to me that many people probably have been viewing these videos at the wrong aspect ratio.

What am I doing wrong, and how should I do this right, to ensure the viewer sees what I am seeing? Can someone give me a simple workflow that I can tape to my wall? I'm frustrated at my own inability to learn this and keep it in my brainpan, but there it is.