Mack Trucks will build a plant in Winnsboro, S.C.

The nation’s second-largest heavy truck maker, said the assembly plant will cost about $80 million and will replace one that has operated in Pennsylvania for 60 years, state and company officials reported. The plant in the central South Carolina community would employ about 1,200 people, Gov. Dick Riley said. Company officials told him construction will begin next month, with the plant to be operational by July, 1987. Mack announced plans in October to replace the plant in Allentown, Pa., which employs about 1,800 people. Mack had left open the option of building the plant in the Allentown area but demanded that the United Auto Workers accept wage and benefit concessions in return. When officials of the UAW international refused, Mack announced that the assembly factory would be built elsewhere.