The wiring of an opto tends to depend on the load that needs to be driven, power considerations and frequency of the signal.
If I understand correctly the 300V pulse is the actuation pulse for the injector. It's hard to say how much power is behind that, but to be on the safe side it would be best to minimize loading.
I would use a large, 10 MΩ or more, divider to drop the voltage to something manageable. That signal should then be buffered by an op-amp before being sent to the opto-isolator input. The output of the isolator can then go to the LabJack. To ensure proper isolation across the opto the op-amp needs to be powered from a source other than the LabJack's. That's the simplest way I can think of to do this properly.
If you want to go an easier route you can put a 1MΩ resistor between an IO and the CDI signal. There are a couple risks here. The signal may be overloaded and impede the operation of the CDI. Or the ignition noise might interfere with the LabJack's operation or maybe even damage the LJ and connected equipment.
Also. If the engine has a crank or cam sensor it might be easier to monitor that signal.