he site was Jerry Ford's idea, a river-front panoramic view of the city where he grew up. He left to the architect the design of the new $7.1 million Gerald R. Ford Presidential Museum, specifying only that it be functional and attractive architecturally.

"Marvin DeWinter the architect says he had the idea to make it a triangular building one night when he was folding a cocktail napkin," according to Will K. Jones, the museum's curator.

There are other stories here about why the 43,620-square-foot concrete and glass structure is triangular. One is that there weren't enough memorabilia from Ford's 30-month presidency to hang on four walls. Another: How many footballs can you hang on one wall?

Yet for all the inevitable joking, what has risen on the west bank of the Grand River is a building rooted in the Midwest, as down-to-earth and solid as the man it commemorates. Backed by a freeway, the site fronts on a small park where furniture factories once stood.

"It's a logical design," said Jones. "The eye of the building is on the city of Grand Rapids and the back of it, where there are no windows, is solid, shutting out both the noise and the visual experience of the interstate highway behind."

President Ronald Reagan will join Ford and 2,500 other important invited guests tomorrow in dedication ceremonies here. The list of distinguished guests includes Vice President George Bush and Speaker of the House Thomas P. O'Neill Jr., Lady Bird Johnson, Margaret Truman Daniel, John S.D. Eisenhower, Sen. Edward M. Kennedy, Happy Rockefeller and Henry A. Kissinger. Several foreign dignitaries will attend. Scheduled to be among them are Canadian Prime Minister Pierre Trudeau, Mexican President Jose Lopez Portillo, former French president Vale'ry Giscard d'Estaing and Japan's minister of foreign affairs, Masayoshi Ito.

At a press conference this week, Ford said that neither Richard M. Nixon nor Jimmy Carter had been invited. "We wanted to concentrate on my presidency and we thought two former presidents would undercut that," Ford explained.

But it is Jerry Ford's week-long presence here that has most excited the people among whom he and his wife Betty once lived. They know the story of how he became president, but suddenly they are beginning to realize the historic significance of his time in office.

A commemorative committee raised $11.5 million from private sources (including $1 million from the government of Japan, $200,000 from Saudi Arabia, $100,000 from the late shah of Iran). Of that amount, $3.4 million was spent on the Ford Library, which houses his presidential papers at the University of Michigan in Ann Arbor, where he went to school. The library was dedicated this spring.

"When you see the exhibit on something like the Mayaguez," said one young local journalist, "you realize our Jerry had his finger on the big switch. It's an awesome thing. He may be the last honest man in America."

The first thing to confront museum vistors as they enter is an excerpt from Ford's first presidential speech inscribed on a stone wall:

"You have not elected me as your President by your ballots, and so I ask you to confirm me as your President with your prayers . . . I have not sought this enormous responsibility, but I will not shirk it . . ."

From then on, the tour is a story of the 38th president, whose first political campaign began in a Grand Rapids Quonset hut in 1948 when he first ran for Congress. There is a reproduction of the hut just as there is a reproduction of the Oval Office, where his career as president ended in 1977.

"The reproduction of the Oval Office brings back great memories," Ford told a press conference here this week, "but under no circumstances would it intrigue me enough to run again."

How to tell that story fell to three Washington area firms: Staples and Charles Ltd., which designed the exhibits; Design and Production Inc., which built them, and Peter Vogt and Associates Inc., which produced the 28-minute film titled "Gerald R. Ford -- the Presidency Restored." The film introduces visitors to the exhibition in a ground-floor theater which was the gift of the massive Grand Rapids Amway Corp.

Robert Staples and his partner, Barbara Charles, said that when they were given the task of recreating the Ford presidency through his memorabilia, they were momentarily stumped.

"How do you do an exhibition of 15,000 square feet on a presidency that's only 2 1/2 years long? We just didn't think there was anything," said Staples.

What they were up against was the recently completed John F. Kennedy Library, which had spelled out a period of great social unrest as well as the loss of a great man.

"We were facing a kind of uphill battle. But we thought that Ford was human, warm and approachable and that's what we tried to do here," Staples said. "The building was already concrete and glass and we wanted to bring it down into scale -- to warm it up with little rooms. That's why there's so much wood and fabric."

So the exhibit shows not only a replica of the Oval Office furnished as it was when Jerry Ford occupied it, but also half a table set exactly as it was when Giscard paid his Bicentennial visit to the United States and sat at Betty Ford's right at a dinner in his honor. Place cards beside pieces of the Lyndon Johnson china, borrowed from the White House, show also that actor Clint Eastwood sat on Betty Ford's left and choreographer Martha Graham was on the French president's right.

Other features in the exhibition: a cutaway model of the White House showing the state and private rooms; Betty Ford's blue-and-white wool inaugural dress and two favorite evening gowns she wore on several state occasions; Richard Nixon's letter of resignation to then-secretary of state Henry Kissinger; transition-period suggestions by Ford aide Philip Buchen, which included one that said while Alexander Haig Jr. had done yeoman service for his country (as Nixon's last chief of staff) ". . . he should not be expected, asked or given the option to become your Chief of Staff."

What probably was the most significant and controversial action of Ford's presidency -- his pardon of Richard Nixon -- is also displayed. Two typewritten pages signed by Ford define his action. The pen is mounted, too. Also part of the exhibit is the copy of Ford's speech in which he explained his action to the American people.

Another display is titled "Three Days in the Presidency" and recounts the May 12, 1975, Mayaguez incident, in which Cambodian troops seized a privately owned American cargo ship. At the end there is an explanation titled "Aftermath of Crisis." Ford's military rescue of the 40 members of the crew on May 15 in the Gulf of Siam resulted in the deaths of 15 men in battle and 23 in a helicopter crash in Thailand, three missing in action and 50 wounded.

"I thought it very important to have the information out front. It's just the way it was," said Nancy Growald Brooks, who wrote the text. "There never was any wish expressed by President Ford or anyone else discouraging the use of the figures, or to mask the terrible cost of the rescue operation."

Ford himself said during a recent interview that the exhibits didn't present everything the way he might have done it had he been trying "to put the best light on what I did or what happened. But for historical purposes, it was important to show it as it was." But in his press conference this week, Ford said he was "totally overwhelmed" by the final effect of the museum.

"You can't see all of this without being emotionally affected," he said.

One of the most poignant of the displays is a slide show on the American Bicentennial put together by Barbara Charles. It is made up of the everyday, anonymous faces of America. The Bicentennial was the birthday party for America that occurred while Ford was president but neither his name nor face ever appear in the slide show. Background music is an original score by composer William Bolcom, played by the University of Michigan band.