The 'aspect ratio' in dosbox doesn't do what you want (i was confused too). It is 'merely' changing the form of pixels during scaling up to be more like CRT pixels (which are rounded) not like LCD (square). I think they have different height too or something. But the most common form of distortion actually comes from the graphics drivers scaling 4:3 to 16:9 (or whatever), making everything look wider.
If you want to disable stretching you have two options depending on what you need to do. For 'most' dosgames you can do:
fullscreen=desktop
and
output=opengl or openglnb or ddraw (depending on what you have and if you want blur or not on the case of openglnb). These outputs stretch to fit which in the case of 1366x768 and most dos resolutions means a almost-square without bottom bars centered on the screen.
Then there are some games and most importantly, windows 95, which confuse dosbox with resolution changing internally and can screw this up. For those you want to mess directly with the streching of your graphical drivers.
On my box on linux i do this with
first set you game conf file
fullscreen=original (this causes windows internal resolution changes to be picked up by the driver)
have a 'streching output' configured too, then on the command line:
xrandr --output LVDS --set "scaling mode" "Full aspect" (this causes the mesa driver to strech 'centered' - there are a few more options you can search in google and beware that LVDS is a monitor id so it can be different for you). This command will reset on boot on linux unless you take the trouble to find the right config file or put it in rc.local (i didn't).
This might or might not get get blackbars on the bottom. For me it did, slightly, but it's already a hell of a lot better than before. Of course if you're using a different driver/not on linux, it will be different for you, but the idea is the same.
