Medco Containment Services Inc. of Elmwood Park,...

Medco Containment Services Inc. of Elmwood Park, N.J., and National Data Corp. of Atlanta agreed to a merger that would create one of the nation’s largest health-care and computerized sales processing companies. The deal, a stock swap, was valued at about $425 million. Medco provides low-cost prescription drugs for employer-sponsored health plans. NDC provides credit authorization services and computerized pharmacy-management equipment. Executives said the merger would allow the new company to expand the potential for credit card payment systems in the health-care industry.