I'm using a 4:3 CRT on a PC with a 8800GT, and I'm trying to reproduce the exact same look I would have with DOS. When I boot MS DOS on that computer, I have a perfect, non-stretched, image that fills my monitor.

Here's a close up of another game with a checkerboard pattern, showing no signs of upscaling:

In Windows, the lowest resolution I can change to is 720x400 (non-square pixels), with​Display Changer. Whether I use Display Changer or OpenGL mode to set the resolution, I end up with an image that is too thin, or even thinner:

Using OpenGL mode and ctrl+alt+(plus or minus), only the orange background is stretched fullscreen, the buttons, menus and the game itself are still of the incorrect aspect ratio.