The men were flown to Allentown from Chambersburg Pa., Hospital, officials said.
Paul Shannon, deputy chief of the Marion Fire Department, said the cause of the fire is unknown.
"We don't know what was in the tank," Shannon said.
Shannon said the men were working with a power saw and a welding torch. He said he didn't know which tool was being used when the fire started. He also said he didn't think there was an explosion, only a fire.
Stull and his son were working at the Marion Auction Service at 5174 Horst Road around 4 p.m. when the accident occurred. They are not employees of the auction, said Jerrold Oaks, owner of the business.
The auction sells farm and other equipment.
The men had bought some old metal equipment at the auction and were cutting it into 3-foot lengths to haul to a recycler, Oaks said. He said they had two trailers nearly full of scrap metal when John Stull started to cut into the tank.
Oaks said it was a "saddle" tank - the type used on tractor-trailers. He said gasoline could have been mixed with diesel fuel in the tank.
Oaks said he was about 500 yards away when he first saw the smoke. When James Stull saw that his father was being burned he ran over to help and the tank flared up on him too, Oaks said.