If you've made a PAL cap at 720 x 576, and you just want to keep the middle 480 lines for NTSC, the aspect ratio will be wrong. Call me stupid, but I don't understand how you can "not use the full width" of 16:9 without scaling the picture. I'm willing to be proven wrong, but the three seconds of thought I've given this suggest I'm not.

Non-ana 2.35:1 PAL is about 330 lines; ana NTSC 2.35:1 is about 360. So if you scale up just 10% vertically, you'll be right on the money in terms of aspect ratio, with a very small amount of scaling.

I'd also take issue with the claim that "it makes a big difference" to the image quality by not scaling. I think Moth3r's captures (which you've all seen) and my own (which you haven't) refute that pretty solidly.