5-Ton Head From Easter Island Is Put on a Pedestal By GRACE GLUECK A team of stoneriggers using sledgehammers, crowbars and grease (the industrial and elbow variety) succeeded yesterday in cajoling a five-ton stone head from Easter Island onto a pedestal on the Seagram Building Plaza.      The operation took about two and a half hours and, long before it was over at 2:10 P.M., a crowd of nearly 300 Invited guests had deserted the windswept plaza at 375 Park Avenue at 53d Street for drinks and speeches inside the building.      Edmundo Lassalle, a speaker at the dedication ceremonies, remarked with a smile "I'm surprised that the head, which was carved by hand, transported by hand and set up by hand on Easter Island centuries ago, should have taken nearly as much time to install in this day and age as it probably did then."      Mr. Lassalle is chairman of the International Fund for monuments, which, with the Government of Chile, is sponsoring the head's appearance here to call attention to the neglected state of Easter Island monuments and to help raise funds for their restoration.  The head will be on display until Nov. 21.      The installation was delayed an hour when the Stone Derrickmen and Riggers Union protested attempts to install the head without the presence of union riggers.  Some members of the union, working on a nearby construction site, noticed the proceedings on the plaza and summoned George T. Moran, a union business representative.                      A Severed Head      Mr. Moran persuaded five riggers from the construction site to donate their services during their lunch hour.       The head and its specially designed, 12foot steel pedestal were lifted on to the plaza Site by construction crane operators.      The difficulty in the installation came about because the 41/2foot by 3foot, well sunk into the pedestal top to grip the head was only within a few inches' tolerance of the masonry bonded to the head's base.      The holloweyed, purse-lipped image, which many observers thought looked like Richard Nixon, was carved of volcanic rock. Eight feet high, it was split from a 12foot torso in 1960 by a tidal wave that ravaged the tiny south Pacific island. which is 2,350 miles from its owner, Chile.      About 1000 of the massive statues, once arrayed on long stone altars but now mostly :cattered on the ground, remain on the island. They are thought to represent ancestor images, carved from A.D. 850 to A.D. 1680 by a culturally sophisticated group of inhabitants whose origins are shrouded in mystery.          'Violates' Cultural History     "Easter Island violates some of the most basic rules of cultural history," said Dr. William Mulloy, a University of Wyoming archeologist who has been excavating there over 13 years.  "In this isolated spot culture developed that was almost on the verge of civilization.  The why of this is the most important thing we can learn from the island."      Dr. Mulloy is supervising an Easter Island restoration program, initiated eight months ago by the International Fund for Monuments and the Chilean Government, which was represented at the ceremonies by Domingo Santa Maria,  Ambassador to the United States.      Money to bring the head here by plane was donated by Mrs.  George Staempfli, a New York art collector, in memory of her father, George McFadden, and her two brothers, George P. McFadden, and Alexander B. McFadden.      The exhibition, including a related display of Easter Island artifacts in the lobby of the Seagram Building, was arranged by Samuel Adams Green, special projects director for the International Fund for Monuments.