Noel Jones was 18 when he was arrested over the murder of 15-year-old Janet Commins in Flint.
He admitted manslaughter and served half of his 12-year sentence but has never challenged his conviction.
Stephen Hough, 58, is now on trial at Mold Crown Court accused of Janet's rape, sexual assault, murder and manslaughter but denies the charges.
During cross examination by Mr Hough's defence barrister, Patrick Harrington QC, Jones said the contents of two confession statements he made in 1976 had been made up by officers and he was pressured into signing them.
Mr Harrington said: "You are saying you didn't say the things that were written down?"
Jones: "No sir."
Mr Harrington: "Was that the case with both of the statements?"
Jones: "Yes sir."
Mr Harrington: "So these are just things that were made up were they?"
Jones: "Yes sir."
Mr Harrington: "... to frame you for a crime you hadn't committed?"
Jones: "Yes sir."
Asked if he realised he was signing a confession to killing someone, Jones said he "didn't understand it quite at the time".
Mr Harrington read sections of the signed statements, which give detailed descriptions of the attack, including where Jones allegedly marked the spot on a map where he "caught up with" Janet.
"Where's all this come from Mr Jones?" Mr Harrington asked.
Jones: "I don't know sir."
"This statement... it's not me... I never done this thing," he continued.
The court also heard from Mr Hough's former wife Delyth Sands, who was married to him between 1982 and 1989.
Giving evidence from behind a screen, she described a conversation she had with her former husband while living in Germany, where he was serving as a soldier.
She told the court: "He said that he had killed someone and I said 'well, who have you killed?' and he said 'someone back home'."
When she continued to question him he became "quite aggressive" and told her not to ask again, she claimed.
The court heard Mrs Sands only told police about this conversation 13 days before the trial started.
It has previously heard that DNA extracted from samples from the murder scene 40 years ago matched Mr Hough's DNA profile.
The trial has been adjourned until Thursday, when the jury will be taken to Flint on a site visit.
A man who was jailed after he admitted killing a schoolgirl in 1976 has said his confession was made up by police.