The 71-year-old was killed, along with two other spectators, during the Jim Clark Rally in the Borders, when one of the cars veered off the road and ploughed into them.
Family, friends, and former colleagues of Mr Stern, from Bearsden, will pack into Clydebank Crematorium on Tuesday morning.
Camper van enthusiast Mr Stern's hearse is expected to be followed by a convoy of the vintage vehicles.
Businessman Iain Provan, 64, his partner Elizabeth Allan, 63, from Barrhead, were also killed.
A fourth spectator struck by the car was left fighting for his life. In a statement, Mr Stern's family said: "Len was a special uncle who was well-loved by the family. His death is really tragic but he died watching the sport that he loved."
Concerns were later raised that the "dangerous" road where the group were hit was previously closed off to the public over safety fears.
The families of two other spectators who died during the crash at the Jim Clark Rally also released tributes to their loved ones.
Mr Provan's relatives said: "The family would like to thank race officials, members of the public and the emergency services for their assistance following the tragic incident. We would also like to thank everyone for their kind thoughts."
The family of Mrs Allan paid tribute to her. In a statement, they said: "Betty was a caring and loving daughter, sister, aunty and great aunt and will be missed immensely."
Meanwhile, police will carry out a review of safety at public events in Scotland following the deaths.
Justice Secretary Kenny MacAskill announced the Scottish Government will also commission a review of motor sport event safety in light of the tragedy. He said: "Spectator safety must always be paramount."