I have several trucks with lift kits from suspension lift company's and every one I have received are narrower just like you are describing. The new springs are narrow but the bushings that are in the end of the spring usually are the correct width to fit in the shackle or spring bracket. You should not have any problems as long as you ordered them for the right make and model.
I am not sure if the weight rating is the same or not. The new springs are arched more and probably have more springs than the original factory so I would think it would be the same or heavier. Hopefully this helps, some other guys will chime in if this is not satisfactory.