A proposal by the National Park Service would allow no more than 330 snowmobiles a day into Yellowstone National Park during the winter - a significant reduction from recent years.  The agency had been studying several options, ranging from 720 snowmobiles a day to an outright ban as it wrestles with the long-running dispute between environmentalists and snowmobile enthusiasts.  During the past two winters, officials have allowed 318 snowmobiles and 78 multi-passenger snow coaches a day under temporary rules. In previous years, as many as 720 snowmobiles were permitted.  The proposal released Thursday would allow the 318 snowmobiles and 78 snow coaches for one more winter, with the roposed changes taking effect in the 2012-13 winter season. All snowmobilers would still be required to arrange for guides.