A feared hit man for John Gotti pleaded guilty Thursday to arranging a decades-old slaying of a Staten Island businessman the Dapper Don suspected was a rat.

Joseph Watts admitted plotting to whack Fred Weiss, a mobbed-up, trash-hauling exec and former Staten Island Advance editor who Gotti thought was cooperating with the FBI.

"I conspired and agreed with others to murder Fred Weiss to prevent him from cooperating with the government," Watts told Federal District Judge Colleen McMahon.

Watts, 69, faces up to 13 years in prison when sentenced in April. Prosecutors said that Watts put together a team of killers to take Weiss out.

He narrowly avoided one ambush by a gang of hit men, but was killed the next day when a second team caught up to him.

One-time acting Gambino crime boss John (Jackie the Nose) D'Amico pleaded guilty in August to conspiring to kill Weiss and was sentenced to three years.

Watts has previously served time for tax evasion and for helping dispose of the body of a Gambino victim. He also allegedly played a role in one of the most famous gangland killings ever - serving as a backup gunman in the slaying of Paul Castellano in front of Sparks Steak House, propelling Gotti to power.

Lawyers for both sides were prepared for a long trial that would have replayed some of the goriest moments of the Gotti-era.

But Watts chose to plead guilty after the judge allowed prosecutors to present evidence in 11 killings Watts was suspected to have played a part in, including Castellano's.

schifrel@nydailynews.com