i dnt see how salt makes you gain body fat. Body fat results from consuming more too many cals. I dnt know if low salt intake is causing the cramps (~RDA is bout 8g/day i think) i would probably go and see a doctor about it.
If your drinking enough water you really shouldnt have to worry to much about your salt intake. I would imagine that you are getting more than 800mg a day from your food anyway most meat and fish has more than 800mg per 100g.