If what Jorge suggested does not work (it may very well do so), the things that I would try would be:
1.intensify your negative with selenium toner. Since selenium is proportional to silver density it will increase negative contrast. I have used selenium in dilutions of 1 to 3 for this purpose.
2. If that doesn't produce enough contrast the next step would be to create a low density unsharp negative mask of your camera negative. This mask would be printed along with your camera negative. The mask will give you an unlimited amount of contrast increase depending on the density of the mask.