With my B310 Sunny's, I always back off the clutch stop bolt on the pedal box, then pull the clutch pedal up, which will move the hook on the top of the pedal. You should be able to slip the cable end over the hook. you then adjust the stop so the clutch pedal is the same height as the brake pedal & adjust the clutch cable at the firewall. You can also change the clutch cable to one from a 120Y or a Sunny, which have the larger body needed to fill the hole in the bell housing. The thickness of the washer could be just enough to make it impossible to get the cable on the clutch fork.

Have you swapped from an A12 to an A14 or A15? The larger engines have a thicker flywheel, so the distance between the gearbox face & the clutch fork when the throwout bearing touches the pressure plate forks is greater. I always used the A12 flywheel, as it's like fitting a lightened flywheel. Are you're using the plate between the engine & the gearbox? Removing this will move the gearbox forward & should help with fitting the cable.