The head of Pan American World Airways' pilots union said the union has reached agreement with the company's board on a package of $58 million worth of wage, benefit and work rules concessions by the pilots in exchange for about 7 million shares of stock to be distributed over a five-year period. Capt. Archie Lenzi, chairman of the pilots' Master Executive Council, said the pact is subject to ratification by the membership. The pilots union is one of four with which Pan Am Corp. said earlier it expects to agree on concession packages. Officials of the troubled airline maintain that concessions totaling at least $180 million are needed.