Nissan Motor Manufacturing Corp. announced here today that it will build its $300 million, 2,200-employee truck assembly plant in Smyrna, Tenn., just outside of Nashville.

According to Takashi Ishihara, president of Nissan Motor Co. Ltd. (Japan), the investment is the giant automaker's largest ever outside of its home country.

Marvin T. Runyon, until recently a top-ranking Ford Motor Co. manufacturing executive and now head of Nissan's American manufacturing operations, said the choice of the site was made only Monday. The cost of transportation to and from Nashville gave that city the edge over Atlanta in the final decision, he said.

Runyan said the Great Lakes region and sites in the Southeast had been considered earlier, and the possible sites were narrowed down to three places, all in the Southeast, a few months ago. The facility will produce an estimated 120,000 trucks a year, using imported, Japanese-made power trains and other parts, Runyan said.

About 35 percent of the finished product will be made in the United States initially, he said. California and the Southeast each account for approximately 35 percent of Datsun's sales, Runyan said.

The American plant will manufacture three front-wheel-drive versions of the Datsun pickup: the standard one, the 4-by-4 and the King Cab.

The Nissan executives said ground would be broken for the new plant within a couple of months and full production would be expected by the fall of 1983.

Ishihara read his initial statements in English and then took questions, with the answers quickly translated from Japanese by an aide in this unusual press conference. The Nissan executive said Datsun trucks account for roughly 20 percent of Nissan's u.S. sales. He said the imposition of a 25 percent import tax on commmercial vehicles has not yet hurt Datsun truck sales but that he and other Nissan executives expect the tax to have an effect sometime soon.

Whatever happens on the tax front will not change the Japanese automaker's plans, Ishihara said. The U.S. truck assembly plant will be built whether or not the American government begins some sort of import restriction, he added.

Asked if presidential aspirant Ronald Reagan's ranking foreign affairs adviser Richard Allen, "who's on your payroll," had any influence in the decision about the plant location, Ishihara said no. Through his interpreter, he said Allen was an employee of Nissan's wholly owned U.S. subsidiary and not an employee of the headquarters Japanese company.