Don't switch the leads, it will fry the fuel gauge. When I hook my 30 OHM fuel gauge to a sending unit that is not matched for the gauge, a 78-10 OHM for example,the needle moves backward as you describe, that is why the two units need to be matched in OHMS. I have been troubleshooting my own gauge and discovered it was grounding its posts to the holes in the gauge bucket so be sure when you tighten the posts to the housing that they do not touch the bucket.
http://s23.photobucket.com/user/littlebuddytoo/media/48Chevy/Bucket.jpg.html I had to put a tiny bit of shrink tube at the bottom of the post to keep it from touching the other metal. I would say, check and recheck your grounds also.
