First it was the 15-to-1 Spirit Level that took a shot at Seattle Slew in the early running, and then it was second-choice Run Dusty Run that made a big challenge at the far turn, and then Sanhedrin threatened in the stretch. And at the finish the untroubled Seattle Slew had won the Belmont Stakes as he pleased.

When did rockey Jean Cruguet know he had the final piece of the Triple Crown won with Seattle Slew? Every inch of the way. He got the Slew in front as soon as possible, set a slow pace that conserved his mount's energy, shook off every challenge with some simple clucks to the Slew, and was four lengths the best of Run Dusty Run at the end.

Three jumps from the finish, the jubilant French rider was high in the saddle, waving a free hard to the 70,229 fans in the stands, the 30 million watching on television, and some part of the 50 million Frenchmen who were watching this race on the tube. It was the easiest of the Slew's Triple Crown triumphs.

As if to case the joint, Seattle Slew came out of his barn at 7:30 this morning with exercize boy Mike Kennedy astride. Mostly, he just horsed around in the backstretch with Trainer Bill Turner nearby, mounted on a stable pony. But later the Slew took a little matutinal gallop past the empty stands that would be filled with screaming thousands 10 hours later when he would be running for keeps for that same stretch.

The first genuine surprise of the day occurred when the gates opened to the public later in the morning. They were seeing sedate Belmont, the prim and proper track, with a new face. Somebody was making an attempt to humanize it. A wandering minstrel bond, The Bano Rascals, was beating out foot-tapping tunes at the entrance, including "Roll Out The Barrel," and behind the clubhouse another group calling itself "Up With People," was giving out with thumping rock. In mass agony, the ghosts of August Belmont, and Ferdinand dePeyster Vanderbilt and the Widener Smythes swooned en masse.

An early development was the scratching of the absurdly named Johnny Camp colt, Hey Hey J.P., which opted to race at Keystone Park today instead of contesting Seattle Slew and his crowd. The Belmont would scarcely feel the impact of his withdrawal. In his last 14 races. Hey Hey J.P. was 0-for-14 and finished 24 lengths behind Slew in the Preakness.

There was some speculation that Se- attle Slew might not be able to handle an off track as well as the fast ones, and the tote board in the infield was saying, in lights, "Muddy." But that description was a rotten misnomer. It was muddy only in spots, and the times of the early winners today were suggesting an absence of mud. In the fourth race, at seven furlongs, the three-quarter mile post was hit in a sparkling 1:10.2, very fast time.

One of the questions being asked was which horse, or rider, would choose to go with Seattle Slew in the early running today. For The Moment in the Derby and Cormorant in the Preakness found out that this was disaster, yet the field today couldn't afford to let the Slew set a leisurely pace for himself and then blow them all out of it in the stretch.

Even his won rider, Jean Cruguet, had some reservations about letting the Slew go too fast in the early running. "I need to hold him to a 47 second-half mile, and then we will be O.K." Cruguet said in his best Parisian English.

Cruguet was riding races earlier in the day to familiarize himself with the condition of the wet track. He picked up $75 for winning the first race, but a paltry $25 for finishing out of the money in a later race. But he stood to win a nice 10 per cent of the purse, or $11,000, for his 2 1/2 minute ride on a winning Seattle Slew two hours later.

The crowd was announced in mid-afternoon as 70,229, a splendid turnout and the second largest in Belmont history.Track officials were exploiting all that betting money by spacing out the usual 30 minutes between races to get the people to the windows.

Sanhedrin, which with Run Dusty Run, is viewed as one of the late-stretch threats to Seattle Slew, had his regualar rider, Jorge Velasquez, back in the saddle after a three-week absence. Velasquez, who went down in the same spill that put young Steve Cauley in the hospital, had less lead in his saddle because he was wearing a 2 1/2-pound iron brace to prop up a fractured ankle.

The mystery horse in the race was Penny Tweedy Rindquist's Spirit Level, which has been avoiding Seattle Slew all year, but gained some credentials when he beat out Sanhedrin in the Peter Pan Stakes here at 1 1/8 miles 10 days ago. Like the Slew, he is a front runner and was viewed as the one who might tackle him in the early running.

As Belmont, time neared, there was a bit of a dither in the outdoor saddling area behind the clubhouse. All of the other steeds, eight of them, were in their area under saddle, but Seattle Slew was a conspicuous absentee. What was keeping him? Was he deliberately trying to psyche his opposition.

The schedule was now cock-eyed. The "riders-up" command, supposed to be given at 5:30, was withheld until the Slew showed up. The 5:40 post time was threatened. Finally, the Slew showed, with his handlers. The explanation was that parked cars in the barn area had forced him to take a five-minute detour. This was believable, and post time was set back to 5:45. To the Slew, it would make no difference.