Way to convert 960X540 video to 640X480?

I'm working on a video project where the original video came from a mini-DV recorder (640X480).

After editing the video in iMovie 10.1.8, I have been able to arrive at an edited 960X540 16:9 Quicktime movie with black bands on the left and right. Is there an app that will strip out the black bands and revert this to 640X480 4:3? I have reason to think the 540p 16:9 file isn't going to give me the best results moving forward.

Longer story:

Not thinking very far ahead, I imported the video from the mini-DV recorder into the current version of iMovie (10.1.8). I have a retina iMac on High Sierra (10.13.2). I'm not quite sure what iMovie did to the original source video, but at some point I made it fit the vertical height of the video to the 16:9 aspect. That gave me black bands left and right, and a central image in the original 4:3 aspect ratio. (Not a problem by itself. Although, if there's a way to tell iMovie 10.1.8 that I don't WANT a 16:9 video, that might also give me what I think I want.)

The video project was edited (sliced, diced, rearranged, music added, stills inserted, transitions applied, titles layered on) to our satisfaction. (I have a less knowledgeable partner who relied on me not to screw up.)

Thus, we were almost ready to turn it into a DVD (yes, a DVD, just like in the old days).

Problem #1: iMovie no longer provides for inserting chapter markings that can be made part of a DVD's menu. (Uh oh.)

After reading the reviews in the App Store about alternative chapter-adding programs and alternatives to burning a DVD with iDVD, I concluded that the simplest thing we could do was export the movie to a Quicktime movie that I could take into the old version of iMovie, which I still have on a Mac Pro G5 (PPC) dating back to at least 2005. In the old iMovie, we could add chapter marks and then send the movie to iDVD. Simple, right?

Problem #2: iMovie doesn't seem to offer me to the option to export the movie as anything less than 540p (960X540, 16:9).

When I import that into the old iMovie, the video then appears with black bands on the top and bottom in ADDITION to the black bands on the top and bottom. (Grr.) I could be mistaken, but I have the suspicion that when this is sent to iDVD, the video will be converted to a format that has only 480 pixels top to bottom, and that the black bands on the top and bottom will STILL BE THERE, meaning that some resolution from the original 640X480 will be lost. The images look bad enough already: I don't want to lose any more resolution than I have to.

So the question is, is there any kind of app out there that can give me a 640X480 4:3 video file (retaining the original aspect and most of the original image quality, such as it was) either from iMove 10.1.8 or from the 960X540 16:9 Quicktime movie that I got from exporting the edited video from iMovie 10.1.8?

I have a copy of Quicktime Player 7, which would seem to allow me to export the video in different sizes, but I can't seem to find the right settings that give me what I want. (For example, I asked for 640X480 in one test and it gave me something like 640X326 instead.)

I briefly looked on the App Store for video convertors, but could not find one that said it would do what I think I want.

I should have used the old Mac Pro to edit the video from the very start, but I wasn't thinking ahead to the DVD part. Now it would be a huge loss of time to re-edit the original footage.

Anybody have any suggestions?