PD can only produce to fixed aspect ratios, so you will need to use a free third party app like VLC or VirtualDub2 to get the specific AR you're looking for. I'd recommend setting the project's AR to 21:9 and then creating a custom 448x192 profile to match the 21:9 AR. (see below for more info)

When you produce to that custom profile, the produced clip will have two horizontal black bars - one at the top and another at the bottom of the screen - so you'll then need to use one of the methods in this discussion to trim the unwanted 32 pixel lines from the top and from the bottom to get a 448x128 finished clip.

Without knowing anything about your source clips, I've created a custom 21:9 profile using AVC/H.264 at 30p.

You can simply download the attached file then open it in Notepad and copy the entire contents. Next, paste them at the end of the Profile.ini file that's located in your PD20 folder (which should be C:\Users\[your user name]\AppData\Roaming\CyberLink\PowerDirector\20.0\UserConfigure)

Please let us know if you need any assistance with creating a different profile or when using VLC or VirtualDub2. It would help if you could attach a short clip from your expected source so that all settings can be adjusted to match as needed.