If you can't find your cable box's exact model number, I would recommend using the closest cable box model you can find in the list. In this case, yes, I would try the DCX3400M that you found.
What you're going to look for is if the virtual remote works. If it does, the model you selected from that list should work just fine.
In the case that you find yourself without a working virtual remote for your Slingbox, here is how you can get back to that list so you can select a different model cable box:
Hope this helps!