After three years of controversy and expenditure of $1.9 billion for the Divad antiaircraft gun, the Army is holding discussions with its builder, Ford Aerospace & Communications Corp., on transforming it into a gun-missile combination.

"It is clear now the real answer is a missile-gun combination," said Undersecretary of the Army James R. Ambrose in an interview.

A long feud has gone on in the Pentagon, out of public view, between civilian officials and military men who favored a mobile gun system for air defense on the front line, and other officers and officials who believed that missiles should be used, a Washington Post investigation has found.

Ambrose said "the long-running battle between the [Pentagon's] gun and missile people" may be behind many of the problems that have developed over the troubled $4.5 billion Divad program.

Privately, active and retired officers acknowledge the guns-versus-missiles battle often is more bitter inside the Army, where budget decisions can determine the careers of the officers running the Army Missile Command at Huntsville, Ala., and the gun backers at the Air Defense Command in Fort Bliss, Tex.

Four years ago, during the field testing shoot-off with General Dynanics Corp. for the Divad contract, Ford protested to the Army that the General Dynamics prototype had Stinger missile launchers along with a gun system. The Stinger is a heat-seeking, shoulder-fired antiaircraft weapon for troops.

General Dynamics had built its Divad prototype with Stinger launchers to enable it to hit aircraft at longer ranges. When the company, according to a former employe, told the Army Divad project manager in 1979 that it was putting Stinger launchers on its model there was no objection.

However, shortly before the trial on the test range at Fort Bliss, Ford learned of the missile provision and protested to the Army.

The Army then prohibited General Dynamics from demonstrating the Stinger, although the system had been integrated into the model on the test range.

Charles French, a retired civilian Army employe who was the civilian technical adviser at the shoot-off, said that the "decision surprised me. I thought they were looking for the better weapons system." He said he remembered that the Army made its decision "on a legal point" that the missiles "were not within the scope of the contract."

Army officials, including Lt. Gen. C. Louis Wagner Jr., the Army's deputy chief of staff for research, development and acquisition, say that the decision was correct because the competition was between gun systems and not a hybrid system with both guns and missiles.

Gen. John R. Guthrie, who had the ultimate responsibility to select the winning system, said in an interview that he had been told that the General Dynamics Stinger was only "a paper proposal," not one available to be tested. He could not say who had given him that information. In any event, he added, "There was no way you could have an equal competition" with one model using the Stinger.

Ambrose, who has been supervising the troubled Divad system, with 30 built and 116 under contract, said he was not aware that General Dynamics had mounted Stinger launchers on its 1980 prototype until he was told that The Washington Post had found that to be the case.

He said that both contractors should have been given the opportunity to have missiles, although, he added, "The Army was dominated then by guns-only officers."