Authorities stepped up the pressure on a barricaded polygamist clan near Marion, Utah, believed responsible for a church bombing. After spending three days well back from the compound, law enforcement vehicles were moved nearer the barricaded gate. Two banks of floodlights were hauled by pickups to points north and south of the log house, and an Army helicopter flew overhead. A light plane had flown low over the compound earlier so police could snap photographs. Vickie Singer, 44, whose polygamist husband, John Singer, was slain by lawmen nine years ago outside the same compound, and Addam Swapp, 27, who reportedly is married to Mrs. Singer and two of her daughters, are wanted on state warrants in the bombing Saturday of a Mormon chapel about a half mile away.