You want your sitbones to be completely ON the saddle, not on the edges over the metal. I would seriously consider the B17 over the B17N. Ask Bill or Diane at wallingford what they recommend. If the bones are just barely perched on the saddle edges you could get some bruising.
The chafing is likely due to the shape of the saddle viewed from above. Pear vs. "T". Sounds like you may be one of the folks who's femur-to-hip-socket angle might require a "T" shape to keep you from chafing at the crease/pantyline/pit.
(compare the top views of the B17N and the B67 for an extreme example of the difference. don't look at width, look at the overall shape)
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