SIOUX FALLS, S.D. (KELO AM) - SculptureWalk and the Washington Pavilion are joining forces in May for the single largest free indoor sculptue exhibit in the Upper Midwest.     SculptureWalk Director Jim Clark says that for two days more than 250 original sculptures will be on display in the Washington Pavilion.     Clark says the works are from the 48 sculptors participating in SculptureWalk.     Clark says many of the pieces, in stone, steel, bronze and wood, will be for sale; sized and priced for any private or commercial setting.