If this is the only negative, that is, this is not something you want to apply frequently in your image-making activity, forget about it and just deal with dense negative, especially if this particular dense neg is important.(posted 8172 days ago)I don't think sepia toning helps to reduce negative density. (It may reduce the refrective density) Furthermore, I think that sulfide solution can very well stain negative's gelatin layer. (By the way, selenium toner is clean working with negative and also very useful as an intensifier)
If negative reduction is something you are interested anyway, I recommend you do some serious library research on it. While doing library research, expose another or two rolls with gross overexposure, and process as if you underexposed. Try every technique on these first. You want to choose one of those classified as "proportional reducer" because you are interested in reducing dense area rather than light areas. If you need to use your ferricyanide bleach, dilute a lot may help. Then partially bleach to the point you like, rinse in plain water, and bring it to your regular film fixer solution. (If you want to start over, redevelop the negative in a regular developer. But reduce very slowly to avoid the risk of over-reducing from the beginning.)