He testified that Michael Hays took the body to Odon, Ind., while Heckathorn helped cut Marshall's truck in two and take it to be salvaged.
Trena Hays, 46, and Brummett, 25, also pleaded guilty Friday and are scheduled to be sentenced Feb. 26.
The Hayses and Brummett face up to five years in prison and a $250,000 fine for concealing Marshall's death, and it's a possible life sentence for growing and selling drugs.
It is unclear how Marshall was connected to the Hayses.
The Hayses and Brummett agreed in court to forfeit $4 million in land, farm equipment, vehicles, cattle and Michael Hays' prized draft horses, bought with money earned from the sale of their crop.
Two Lancaster men who allegedly worked in the farming operation are still scheduled to go to trial. David Miller, 28, and Frank Hall, 28, are set to appear in U.S. District Court in Lexington at 1 p.m. Monday.
A warrant for Frank Hall's arrest was issued by U.S. Magistrate Judge Jennifer Coffman Thursday.
David Miller's sister, Beverly Hall, 27, and his father, William David Miller, 49, pleaded guilty Thursday to growing and distributing marijuana for more than 10 years. A Wisconsin man, Alan Grass, also pleaded guilty to helping on the farm.
They each face 10 years to life in prison and are scheduled to be sentenced Feb. 26.
Heckathorn and William Beasley of Lawrenceburg pleaded guilty in September to federal charges and agreed to testify in the Hays case. As part of a plea agreement, Heckathorn described the marijuana operation.
He said Michael Hays, Brummett, Frank Hall and others planted more than 1,000 marijuana plants in cornfields in Kokomo, Ind., and Odon, Ind., in the spring of 2002. The plants were cloned on Hays' Gravel Switch farm and brought to Indiana in horse trailers.
The families raised the plants until maturity in the fall, when the harvest was dried and bundled and brought by horse trailer to barns in Gravel Switch. Beverly Hall also helped strip the marijuana. Four days after the hiding of Rusty Marshall's body, William Miller shot Michael Hays in the back of the head at a Shell Mart not far from where Marshall's body was found in Odon, Ind.
According to court documents, William Miller told police that he feared for his life, running from the gas station and hiding in ditches. He stopped at a residence to call his wife and tell her to "flee with the children" because she might be in danger.
Miller still faces attempted murder charges in Indiana that stem from the incident.
After Hays was shot, Indiana police found Marshall's body buried in Odon, an agricultural community dotted with cornfields.
An extensive federal investigation ensued, and Hays, his family and the Miller family were indicted seven months later.
U.S. Attorney Greg Van Tatenhove said that the Hays case was "very important" because it was an "organized" criminal drug operation.
"I feel like we've made a good dent in the (drug) problem in (Boyle County)," he said. "I don't believe that it solves the problem for the district, but we've taken a number of individuals off the street, and it sends a message that we are going to be aggressive."
Heckathorn and Beasley will be sentenced Jan. 8.