"WE DON'T change our act -- ever," says Florence LaRue, lead singer for the 5th Dimension, explaining the group's durability after 18 years, surviving personnel changes, turbulent times and no hit record since 1973. The pop group, one of the first black groups to successfully "cross over" to the white pop charts, is still performing nationally. "We never left, never stopped performing," LaRue says, adding that the group is still on the road at least 40 weeks a year. "I just say, thank God I'm working."

In the late '60s and early '70s, the group had a seemingly unerring sense for hit singles: 14 in a row. Their smooth vocals also established several unknown songwriters, including Laura Nyro ("Wedding Bell Blues," "Stoned Soul Picnic") and Jimmy Webb, whose "Up, Up and Away," sent the group's (and his) career soaring, becoming a national catch phrase and winning five Grammy awards.

"Our producer, Bones Howe, was very instrumental in selecting the songs we would record," LaRue recalls. "And Jimmy Webb would sit at the piano and arrange some of the songs for us. Nothing written down -- we'd just work out the harmonies and vocal arrangements live."

"It's kind of difficult to find the same quality of material now," LaRue says. "You know, those songs that you hear and say 'that's a hit.' I'd say it's because all the songwriters now are recording their own material."

Though their homogenized harmonies and smoothed-over soul sound brought them commercial success, it was inevitably accompanied by criticism. "In the beginning, I don't think the black audiences even knew we were black," LaRue says. "There were people who accused us of selling out, who said we 'sounded white.' But we just had to keep on doing what felt right to us. You can't color a sound."

LaRue admits that the group "never really had to pay any dues. We really had a Cinderella story -- our first record the Mamas and the Papas' 'Go Where You Wanna Go' was a Top 10 hit, and all that fairy-tale stuff."

Fashion photographer Lamonte McLemore spotted LaRue in a California beauty pageant while she was still in college and recruited her for his new singing group (originally called the Hi-Fis, then the Versatiles), which included opera singer Ron Townson, Marilyn McCoo and gospel singer Billy Davis Jr., all of whom lived in suburban Los Angeles.

"Our name came from Ron and his wife. He thought of the 3rd Dimension, because 3-D was very popular then. And she suggested the '5th,' for the five of us. So we became 'Your Dimension in Sound,' " LaRue says, mimicking a fast-talking AM disc jockey.

LaRue is reluctant to discuss the split with McCoo and Davis, who left for solo careers in 1975. They were replaced by a series of singers. The current incarnation of the group includes new singers Michael Proctor and Joyce Wright.

A new album on Buddha/Arista records is in the works, with an early 1983 release date. The record will include some standards and some original material, including a tune by the husband-and-wife hit factory Ashford and Simpson, LaRue says.

"We realize the business is changing," LaRue says. "When you don't have a record out, people think you've disappeared. So we're trying to build a career that doesn't depend on a record. Like Frank Sinatra, or Perry Como."

Their attempt to break out of the pop group/oldies mold includes a live cable television special scheduled for this fall, "The 5th Dimension Live at Caesar's." And they've been exploring another dimension recently: the first musical group to do a Broadway musical, performing as the national touring company of the Tony-winning revue "Ain't Misbehavin'."

The 5th Dimension was here last week to perform at a White House state dinner for President Ferdinand Marcos of the Philippines and his wife, Imelda. Their show included most of the familiar hits, plus two special songs. "We added a Duke Ellington tribute for President Reagan, and Morris Alpert's 'Manila Girl,' for the Marcoses, since the song is very popular in the Philippines. We had to wear special gowns, too. I added a piece to my low-cut gown, since the Reagans are very conservative."

LaRue says all the Dimensions work hard at keeping their performances sharp and well rounded. "I am constantly studying voice and dance and movement," LaRue says. "Young performers, rock stars, are so talented, but they don't really work on their craft. They won't have our kind of longevity."