It's hard to judge the negatives with scans to the web. The positive image looks a bit underexposed, but the uninverted image looks pretty good. You are getting an awful lot of density for your exposure time. Stopping down to f/16 may help, giving you more time to work. The higher contrast seems to help, which makes me think that the negatives may be underdeveloped, but not by a lot. The uninverted image of the negative doesn't support that, however, and it may or may not be the case. The short exposure time also makes me think that you may have negatives that are too thin to print well - most likely do to underexposure. Try doing some bracketed exposures and develop normally to see if it really is an exposure problem.