Sugar Ray Leonard and Thomas Hearns will meet at 15 rounds as welterweights, reads the contract, with no mention of any title. Well, one: "The Showdown."

For this little set-to Sept. 16, Caesars Palace will construct a 25,000-seat arena in Las Vegas, and the promoters are talking all-time-high boxing gross of between $40 million and $52 million (including closed-circuit telecast, for which tickets to the Capital Centre showing go on sale at noon today; $25 for the first 5,000 seats, $30 for the rest, all reserved). The reord was $30 million for Larry Holmes-Muhammad Ali last October at Caesars.

With Main Event Productions, headed by rock-concert impresario Shelly Finkel and New Jersey boxing operator Dan Duva, at the promotional helm, formal announcement of The Showdown was made at a New York City media conference yesterday, giving the fighters a chance to posture and jaw at one another.

As for the avoidance of the word "championship in the contract, it's to keep upward of $500,000 of that massive gross from going for sanctioning fees to the rival WBC and WBA, whose 147-pound belts Leonard and Hearns own, respectively. As Hearns' trainer, Emanuel Steward, said with Leonard's adviser Mike Trainer in assent: "The public will know that the winner of this fight will be the true champion. It doesn't need to be written in . . ."

Another 1981 NFL No. 1 draftee picked off by the Montreal Alouettes! It's Keith Gary, the Oklahoma DE selected by the Pittsburgh Steelers, following David Overstreet, the Oklahoma RB for whom the CFL club outbid the Miami Dolphins. Gary, a 6-3, 255-pound Sooner by way of Fairfax's Chantilly High and downstate Virginia's Ferrum Junior College, accepted a one-year Montreal contract with an option year. A $500,000 guaranteed package, says agent Jerry Argovitz. . .

In basketball, American U. guard Robin Hoey gets to practice -- at least -- with the Bullets. The NBA team yesterday made Hoey, AU's MVP last season and a 13.6 scorer, its second free-agent signee (both guards) of the offseason, and given a foot in the door, who knows what the 6-foot-2, 165-pound North Carolinian might pull off?. . . Bigger name in the NBA news: star forward Alex English, retained by Denver under the right of first refusal. "One of the most significant days in the history of this franchise," proclaimed General Manager Carl Scheer in affirming the Nuggets (who last week rejected a San Diego bid for English) are matching the Seattle SuperSonics' bulging offer sheet. . .

Evidently owner Bob Lurie lost patience with the San Francisco Giants spinning their wheels with that 27-32 staring him from the standings all these mornings. Why else fire Spec Richardson in his seventh year as vice president for baseball operations (general manager) and promote the old catcher, Tom Haller, to his post? Indeed, Lurie says the strike "gave me a chance to reflect and review. . ."

The soccer standings in Scoreboard, relayed from the wire services, have been a few points off for some time on several teams, NASL informs -- including Washington and closely pursuing Montreal -- but the Dips lead the Manic by four points anyway, 84-80 instead of 86-82 . . . Anent the Dips not radio-casting this week's two West Coast games, an irony: If a factor was play-by-play man Mike Zimet's would-have-been pay being invested in Johan Cruyff -- here's ailing Cruyff not making the trip either. . .

Broadcasts tentatively in the offing, thanks to the baseball strike: On WEAM-AM-1390, July 14, at 7:05 p.m. in place of the real All-Star Game from Cleveland: a "recreation," says the station ("creation," say we) of a 1981 all-star game by the ever inventive Nat Allbright. . . On WEEL-AM-1310, as long as the strike's stillon: five Alexandria Dukes home games, Sunday afternoons starting this weekend (Salem). Says WEEL's Irv Lichtenstein, "We even have a sponsor for 30 seconds of seventh-inning stretch."

Have you kicked around yesterday's poser, Mike Connell versus Mike Bragg? Their 1980 punting stats almost carbon-copy each other. Gross yardage: Redskins' Connell averaged 39.2, Colts' Bragg 39.1. Net yardage (after substracting return yards and 20 yards per touchbck): Bragg 33.5, Connell 33.4 Bragg's 12th (and worst) Washington season, 1979: gross 38.4, net 34.1. Hang time!. . . Next question: George Scott, erstwhile Boomer of the AL, is batting .385 with 14 HR, 68 RBI for the Class AAA Mexico City Tigers. As one of the most publicized of 1960s-70s players, how many seasons was Scott a .300 hitter? How many times did he eclipse 100 RBI? Hang time.