When President Reagan abandoned his "Dense Pack" basing mode for the MX missile, the House Armed Services Committee was able to cut almost $400 million from the military construction budget.

Almost as quickly, it put $400 million back in--this time for home-town projects sought by members of Congress, not the Department of Defense. The projects, which the Pentagon opposes, range from relocating a gate to ease traffic in South Weymouth, Mass., to rebuilding a municipal fire station in Grand Prairie, Tex.

"Some have greater merit than others, but it's very difficult to judge," said Rep. Ronald V. Dellums (D-Calif.), chairman of the Armed Services subcommittee on military installations and facilities. "In the absence of hard, objective criteria, it would seem you have to trust the credibility and the integrity and the judgment of the members."

By voice vote, the House yesterday approved $8 billion in authorization and a little more than $7 billion in appropriations for military construction. While the total represents a small portion of the administration's requested $280 billion for military spending in fiscal 1984, it offers a choice target for members of Congress looking to bring home federal dollars.

That, in turn, can make life difficult for American forces living in substandard housing overseas, officials said, where no American voters or contractors push for spending. Not counting the change in MX funds, the House this year authorized $186.7 million more than the administration requested for domestic outlays and $288.5 million less than it wanted for overseas spending.

"For the members of Congress, military construction is probably the most useful committee in the House, outside of Transportation and Public Works," one aide said.

Dellums, in his first year as chairman, said his subcommittee handles "very practical matters" as well as policy questions. "That's why I was surprised they gave it to me," he said yesterday. Dellums often votes against military spending.

House Whip Thomas S. Foley (D-Wash.) explained the importance of military spending when he successfully appealed in committee for $22 million extra for Fairchild Air Force Base in Spokane.

"The projects will not only benefit Fairchild, but could also breathe life into the Spokane area's ailing construction industry," he said.

Other representatives relied on less traditional arguments. Rep. Brian J. Donnelly (D-Mass.), for instance, won approval for the $1.4 million relocation of a gate at a Naval Air Station with the help of a letter from the father of a boy killed in traffic near the gate in 1981. The Pentagon does not list the project in its five-year plan.

"The gate relocation is simply not a priority of the United States Navy," Donnelly said. "Mr. Chairman, the gate relocation is a priority for me, and for the parents of David Wood, and for all the parents and residents of Weymouth."

Rep. Martin Frost (D-Tex.) persuaded the House to spend $227,000 for a new fire station away from the Dallas Naval Air Station in Grand Prairie, also a project that does not appear on the Navy priority list. "Noise from NAS Dallas makes it very difficult for fire personnel to monitor their emergency systems," Frost said.

And Rep. Thomas F. Hartnett (R-S.C.), a subcommittee member, successfully proposed spending $1.5 million for a new chapel at the same facility.

"The subcommittee had just previously added new gymnasiums and racquet ball courts," Hartnett's aide explained yesterday. "He thought if we were making all these physical facilities available, why not make something spiritual available too?"

Robert A. Stone, a deputy assistant defense secretary, said the Pentagon will oppose the added projects.

Defense Secretary Caspar W. Weinberger "is under a lot of pressure from all directions to cut spending, and he is really trying . . . to avoid having to cut back further because Congress adds some A10s planes . . . or some construction he doesn't want," Stone said.

Dellums said he will hold hearings to look for objective ways to screen members' requests. But he said that as long as "powerful" members can get their projects through it would be discriminatory to vote against anyone else.