Financial details were not immediately available and the deal is contingent on Allen passing a physical examination to be administered by the 49ers' medical staff.
While the addition of the 34-year-old Allen is a bit counter to the revamping of the roster under second-year coach Mike Nolan, an initiative intended to address the franchise's long-time salary cap woes and to phase in more youth, the 12-year veteran should add experience and guidance to the line. And while there are some who insist that Allen's play has slipped the past few years, he remains a viable starter, and was chosen for this year's Pro Bowl game.
"I think Larry has got a mind-set that every offensive lineman should have," Nolan told The Associated Press on Friday. "I'm looking for an identity that our offensive line is searching for. Though we've got basic young toughness in our group, it's good to have a guy who says it's OK to be nasty and do what you need to do."
The acquisition of Allen by the 49ers was somewhat surprising in that the 49ers weren't among the several teams thought to have an interest in one of the most celebrated linemen in league history. Miami, where former Dallas offensive line coach Hudson Houck works in the same capacity with the Dolphins, was felt by many league observers to be the front-runner for Allen's services. The Detroit Lions were also believed to be pursuing him and there were rumors the Oakland Raiders might attempt to sign him.
"If it wasn't Dallas, I wanted to go back home," Allen, who also spoke to the Oakland Raiders during his brief free agency, told AP. "They offered the best deal, and they're in California, so it worked."
During a career that likely will earn him a spot in the Hall of Fame someday, Allen has played at every offensive line position except center, and is one of just three linemen in history to be chosen for the Pro Bowl at multiple positions. That said, he has garnered most of his accolades at left guard.
San Francisco has a pair of promising guards in Justin Smiley and David Baas, who have just three seasons between them. Smiley started all 16 games at left guard in 2005. Baas started the last five games of the season, all at right guard, but might move to center if veteran Jeremy Newberry doesn't return from chronic knee problems.
The 49ers aren't just bringing in Allen, though, to serve as a mentor to its young linemen. The 49ers lacked inside toughness and strength in 2005 and Allen, even at this late stage of his career, figures to be the team's best drive-blocker in the running game. One element to be watched is Allen's weight, which has been a frequent problem in past seasons, and a point of contention between him and coach Bill Parcells.
Allen is a huge blocker with great girth and, while his weight has been listed at a conservative 325 pounds for the last several seasons, suspicions are that it might actually be 40-50 pounds more than that. Still, at this year's Pro Bowl, the 12-year veteran demonstrated his sheer power, as he captured the strongest man competition, based on how many times he was able to bench press 225 pounds. Early in his career, Allen was so dominant that even some of the NFL's toughest defensive linemen hated playing against him.
In addition to his in-line strength, Allen was adept at pulling in front of the sweep and burying a moving target. He has also been excellent in pass protection.
His long stint in Dallas ended when the Cowboys decided his price tag was too high at this juncture of his career. Allen was due a $2 million roster bonus on April 1 that owner Jerry Jones had no intention of paying, and his release saved the Cowboys about $3.4 million in 2006 salary cap space.
A second-round choice in the 1994 draft, Allen, who played at tiny Sonoma State, has appeared in 176 games in his career and started 170 of them. He started all 16 games at left guard in 2005.
Len Pasquarelli is a senior NFL writer for ESPN.com.