Mack Trucks, based in Allentown, Pa., posted income for the first quarter of $4.6 million, of which $2 million was extraordinary income resulting from the carryforward of income tax credits. As restated because of the company's adoption of a new pension accounting standard, Mack's net loss for the comparable 1986 period was $9.6 million. The company noted that sales for the period totaled $440 million, a 110% increase from a year ago. "The dollar sales improvement primarily reflects the impact of increases in heavy-duty . . . and medium-duty . . . unit vehicle deliveries of 16% and 38%, respectively," the company said in comparing the results to those of a year ago.