M42 is a begger due to it's extreme dynamic range. To get the really fine details you may have to experiment to get your stars really tight before you will see tons of detail in the nebula. M42 is extremely hot to extremely dim way out. Your histogram shows lots of pixels way to the right as you have mentioned you expect due to the 120 second exposures. For high range of intensities you may want to think seriously about something that is a bit complicated but works well, HDR combination at a close to final set of pre-final combined images.
You may wish to consider forming your close to final image using 120 sec exposures (or even more if you want real wispy stuff) then one at 20 sec and then one at 5 second exposures or whatever it takes so that trapezium is still a minimum of the 4 stars all showing (so you may need 3 sec, depends on aperture and focal ratio). I'll be trying that sometime in M42 season as it gets way up high (here in the north +35 DEC area).
After you get each one looking ok for the range of intensity that it is meant to cover then use a high dynamic range multi-exposure tool such as the fairly low cost Photomatrix Pro and your goal is to get a non-burned out center so you have to play with settings of Photomatrix.
You can see a B/W image of this sort of technique on this shot I took a few months back and see the very stretched to the very bright 'sort of ok' I am only learning but this gives you an idea.
http://astrospotter.zenfolio.com/p66...410a4#hd5410a4
Factor this in with the experts on this board as they are WAY WAY ahead of my curve as I am fairly new to this 'art/science' of astrophotograply.
Cheers,
Marko