The GT40R is designed to be used in vehicles fitted with a ballast resistor, like most red-motor Holdens. When you first start the car, the ballast resistor is bypassed, and the coil gets the full 12V(ish). Once the engine fires, the resistor comes on line and limits the current going to the coil (voltage drops back to 8-9V(ish). Ballast resistors were first introduced with the EH Holden.
The GT40 coil is designed to be used in vehicles with no ballast resistor, like the EJ Holden. It operates on the full 12V.
So assuming someone has just plonked a red motor into the 'J, and hasn't added the ballast resistor, it should has a GT40 coil. If you run the GT40R coil with standard EJ Holden wiring, it will soon cook the coil.
Cheers,
Harv (deputy apprentice spitz and sparken technician)