this can be tricky.

Don't connect the pedal end to the cable. Slip a small cord over the end of the fork in the gearbox that pokes out the gearbox to pull up the slack on the thrust bearing spring in the gearbox then slip the cable over the fork end... tie the cord off to the front of the car somewhere to maintain the leverage (to help give you some slack to get the pedal end on)
After you have the gearbox end on then insert the cable through the firewall and you may need to remove the pedal bolt, slip the cable over and re insert the pedal bolt/pin ... all depending on the slack you can get on the gearbox end with that piece of cord.

Remove the cord (cut it off) and you should be right.