After months of tense, difficult negotiations, a plan to spread the effects of anticipated cutbacks on the drought-stricken Colorado River is nearing completion.  On Monday, representatives of the seven states that rely on the river will gather for a formal signing ceremony at Hoover Dam, the real and symbolic center of the Lower Basin Drought Contingency Plan. The plan is a blueprint for shortage sharing built around water levels in Lake Mead, the giant reservoir on the Arizona-Nevada border that has sunk to near-record low levels after two decades of drought.