Vaughn is correct. The only appropriate way to get more contrast in a salt print is to get it in the negative. This process requires more contrast in the negative than any other. The 1840's paper negative that I printed for MoPA a few years ago had greater contrast than any I film negative I have seen other than those made with lith film.
You don't mention your film or developer. You must use a film in the 100-125 ISO range, or slower. Even some of these will not produce the maximum contrast required. Efke 100 is an example of a film in this range which will give you what you need. I use either FP4+ or lith film in camera.
You need to use an active developer. Although I don't use it, HC110 at a dilution of 1+4 from stock should work if you use the times recommended for 1+7 dilution. It will get you close,but time may still need to be increased for the ultimate negatives for this process. ABC, Pyrocat HD, W2D2 all work well, as will your print developer.
So there are a lot of choices from which to choose. Make a choice or two and make tests.
Before I am asked, I do not recommend development times and temperatures because I don't know how individuals meter for exposure, agitate during development, etc
Good luck.