DETROIT, AUG. 21 -- Sales of Cadillac's $54,700 Allante have fallen far short of expectations, with only about 1,400 of the "ultraluxury" two-seater cars delivered to customers this year, a company spokesman acknowledged today.

Production of the V-8 Allante -- the most expensive production car General Motors Corp. has ever offered for sale -- also has been lower than anticipated, with only 3,363 units built for the 1987 model year. Cadillac had said it would build about 6,000 Allantes for 1987.

At the end of July, the Allante showed a supply equal to 179 days.

"That is about three times what it ought to be," said David Healy, an industry analyst with Drexel Burnham Lambert Inc. "That has got to stop some time."

Cadillac spokesman David Hederich attributed the low production to problems at the Detroit-Hamtramck plant, where the cars are assembled after finished bodies are flown from the Pininfarina works in Italy.

"There is no single problem," Hederich said, adding that production has been slowed to ensure quality. "Maybe we were too optimistic. It is a whole new way and there are many new processes to learn all at one time."

Cadillac said the Allante assembly line -- separate from the lines on which the Cadillac Eldorado and Seville, Buick Riviera and Oldsmobile Toronado are built -- has the capacity to build about 7,800 cars a shift. About 120 workers are employed to join the Allante's body with its drivetrain.

Hederich said that about 2,800 of the 3,363 1987 Allantes have been delivered to dealers, which means there are still 1,400 Allantes in showrooms and more than 550 in transit.

Production for 1987 has been completed and the plant is in changeover to the 1988 model year, Hederich said. Two new colors -- black and bright red -- will be added to the current offerings, which are pearl white metallic, silver, gold and maroon metallic.

The front-drive Allante has a detachable hard top and comes with only one option -- a $2,850 cellular telephone. It is not changed substantially for 1988. "There will only be some fine tuning," Hederich said.

Hederich pointed out that although the car was introduced in January to five major U.S. markets, it was not available nationally until April. "Some of the smaller dealers still have not seen the car," he said. Cadillac has about 1,600 dealers.

The Allante's main competitor in what Cadillac calls the "ultraluxury" market is the Mercedes-Benz 560SL roadster, which sells for $59,580. Sales of the 560SL, also a V-8 powered two-seater convertible, have risen to 6,996 units for the first half of this year from 6,873 for the first half of 1986.

"The cars are in short supply and we have no problem selling them," Mercedes spokesman A.B. Shuman said.

Healy said the slower sales of the Allante may be "because the car does not offer anything unique.

"The market looking at prestige automobiles, most of them European, has not been turned on by the car," he said. "If it was a hot car we'd know it by now.'