Exporting videos with a specific, custom aspect ratio. In my case 3840x1070

I'm producing a video that will play on a huge stadium "jumbotron" kind of thing. They require very specific aspect parameters that are very wide. I need them to be 3840x1070. When I try to adjust it in "video properties" it just squeezes the video, don't want that. I can manually crop at the bottom of the inspector and that gives me what I want to do, only approximately. I have to use the crop from top and bottom numerical values. I either need help in doing this another way, or help doing the math to convert an aspect ratio, like the one I need, to values to enter into the "top-bottom-left-right" crop/padding section of the inspector. (there is a dropdown menu with standards like panavision, etc.) There has to be a simpler way that I'm missing. Any help would be greatly appreciated. thx