The board of the Centre City Development Corp. voted unanimously Friday to continue for 60 days negotiations over the future of the downtown Balboa Theater. The corporation has been trying to convert the old movie house into a mixed-use retail complex and modern art museum at an estimated cost of $5 million. In December 1983 the agency approved a 120-day negotiation agreement with the theater owners, but the talks lapsed. A private group called Save Our Balboa has pushed CCDC to preserve the theater as a performing arts center. But on Friday, CCDC Executive Vice President Gerard M. Trimble maintained that an art center would be more feasible financially.