So I did have the game running fine fullscreen but maintaning the 4:3 aspect ratio, then Hellfire came out and now everything is stretched out to fit the screen. None of the video settings seem to make any difference. I tried the belzebub thing, but that gave me regular Diablo running in a small window.

What do I need to do to get Diablo Hellfire running fullscreen and maintaning the original 4:3 aspect?

I had to disable my aspect ratio (21:9) in order to have fullscreen since nothing else worked with various combinations.

My "dxcfg.ini" looks like the following now:

[dxcfg]
display=3440x1440@60
presentation=fullscreen
aspect=disabled
scaling=fit
vsync=application
gamma=1.0
anisotropic=enabled
antialiasing=msaa_8x

However, your issue may be different.

Thanks I was looking for this fix I don't like those black lines

You are welcome.

As a side note with a 34 inch 21:9 monitor, I run into various problems all the time since I like older games (win32 applications) .

Sometimes with other games, I have to go to the:

*.exe (game executable)
Properties
Compatibility
Change High DPI Settings
Override High DPI Scaling Behavior
System (Enhanced)

If for some reason it doesn't work appropriately, I may have to change it to either Application or just System.

(In case you have or will ever have a large monitor & running Win10 with the Anniversary Update/Creators Update, I thought this could be potentially helpful in the future.)

For other people looking for this issue.
Note that your graphics driver might have disabled GPU Scaling, so you might have to activate it.

For example for AMD (ATI) you must go to "Display", "GPU Scaling" -> Enabled. "Scaling Mode" -> Preserve Aspect Ratio:
https://www.amd.com/en/support/kb/faq/gpu-68

Thanks bones82013!

In case you are intersted there is also DevilutionX that lets you expand the view to get real widescreen without stretching or black bars.
https://github.com/diasurgical/devilutionX/releases