Junior Bradshaw, 33, had his 18-year sentence cut to 10 years by three Court of Appeal judges in London.
Kevin Liverpool, 36, who was originally given a life sentence with a minimum term of 10 years and eight months, had his minimum reduced to six and a half years.
The pair, of St Stephen's Close, Longsight, Manchester, were found guilty of conspiracy to murder and conspiracy to rob following a three-week trial at Exeter Crown Court last year.
They drove from Manchester to Miss Stone's home in Devon with an arsenal of weapons to rob and kill her.
Liverpool and Bradshaw were on their way to attack her armed with a sword, three knives, two hammers, masks, gloves and a hosepipe when they were arrested in Cullompton in June 2011.
It had been argued on Liverpool's behalf that the "clumsy and badly-planned" conspiracy was not so serious as to qualify him for a life sentence. But Mr Justice Bean said it was a conspiracy to murder using an "array of weapons".
He added: "When coupled with the finding that the appellant (Liverpool) poses a high risk of serious harm to the public, a life sentence was justified."
As the trial judge had noted, Liverpool was the instigator and Bradshaw was the foot soldier, and "a reduction in his sentences should likewise be made".