The book on Jameel McCline is that he fights not to lose.

That is, the big, affable, quick-handed heavyweight has often backed away from confrontation when his corner has demanded it most. Despite tallying up points and cautious wins with a swift jab and remarkable athletic ability, Mc-Cline – who goes by a showman’s nom de guerre Big Time – has failed to let his heavy hands go in big fights.

Has the reason been a shortage of self-confidence? A soft, fragile pysche?

Whatever McCline’s mental challenges, he must push himself to overcome them Saturday night at Madison Square Garden, where he is a 3-1 underdog against his longtime personal friend, Chris Byrd (37-2-1, 20 KOs). At 34, Mc-Cline (31-3-1, 19 KOs) might be facing his last shot at a title belt.

“This is it for me, I have all my chips in and my cards on the table,” McCline told Doghouse Boxing before going into seclusion at the Olympus Gym, a damp basement down the faded Main Street of Hackensack, N.J. “Listen, if I don’t win this fight I am sitting around waiting for them to call me. And I can’t do that … I might as well hang it up at that point. I do look at this fight as a do-or-die situation.”

McCline and Byrd, also 34, are slated to appear below John Ruiz and Andrew Golota in a night of embattled heavyweights promoted by Don King. McCline is registered to earn $180,000, Byrd $2.5 million. Despite their friendship, the two fighters couldn’t be farther apart in fighting styles and fistic pedigree.

Byrd, 6-foot-1 and 210 pounds, was the youngest child in a religious Christian family of eight in Flint, Mich. Trained by his father, Joe, he won over 275 amateur fights, traveling to countries like Russia, Germany, Italy, and then to Barcelona to capture an Olympic silver medal. The crafty southpaw and IBF champ has since made a name for himself turning big guys like McCline into pretzels with his elusive defense. In Byrd’s last fight in April, however, the slickster showed sluggishness against a reinvented Golota, taking home a draw and two puffy eyes.

McCline, 6-foot-6 and 270 pounds, was born into a broken family in Port Jefferson. He never touched a boxing glove until his release from the Greene Correctional Facility in Coxsackie, N.Y., where he served a five-year sentence for selling guns. Three weeks out on parole, Mc-Cline was tapping a heavy bag in a Staten Island gym when he was spotted by “Doc” Ron Reuther, an athletic goods salesman who took McCline into his home, quickly turned him pro, and managed him briefly.

Size, speed, and natural athletic ability were noteworthy advantages for McCline. But Big Time seemed to suffer big-time nerves. Against the much smaller journeyman Sherman Williams in 2000, McCline, then a top-rated prospect, battled to a draw.Taking his first title shot against Wladimir Klitschko in Las Vegas in 2002, McCline turned stiff from the first bell, failed to throw punches, and suffered an embarrassing amount of punishment. His corner threw in the towel after McCline was knocked down in Round 10.

Sweeping off the ring apron at the Olympus Gym in Hackensack last week, Reuther wondered if the reason McCline had a tendency to show that deer-in-the-headlights look in the ring was because he pushed him too fast. In just his second fight, McCline went up against an undefeated Golden Glove winner who knocked him cold in the first round. Perhaps to compensate for that early humiliation, along with his lack of boxing training, McCline seems to have been searching for that magic formula that, upon ducking under the ropes, will convert him from cautious to killer.

“Jameel will do anything to get an edge,” Reuther said.

A hypnotherapist, a swimming coach, a nutritionist: All have been hired at different points in McCline’s career for conditioning. On some days, the heavyweight practices yoga – a funny-sounding word in boxing’s spit-bucket lexicon – instead of sparring.

But McCline’s penchant for high-tech conditioning and spiritual workouts conspired against him. Before taking on Klitschko, Jameel purchased an oxygen tent designed to give him the benefits of high-altitude training from the comforts of his New Jersey home. But the costly contraption sucked so much oxygen out of the air that McCline had trouble sleeping and suffered exhaustion.

That’s a fight McCline doesn’t like to talk about. (His oxygen tent is currently for sale.)

The other fight McCline doesn’t like to talk about is Saturday night. The McCline and Byrd families have been close for years, staying at each other’s homes, exchanging holiday gifts, chatting on the phone and via e-mail. All communication, especially between Byrd’s wife Tracy, and McCline’s wife, Tina, stopped during training.

“I’m not looking forward to it all,” Tracy Byrd said.