The woodruff key lines the pulleys up so they are in time. The force from the bolt pushes them farther onto the crank which is tapered. I would double check the order of the parts going on the crank in the FSM and what you have going onto the crank and make sure nothing is missing there. If the pulleys were not seating the crank would spin inside the pulleys and shear the woodruff key, the engine would quit because the cams would no longer turn. I've had first hand experience shearing a woodruff key on a CRF450 that I had. Couldn't figure out why it wouldn't run until I pulled the flywheel off the crank. You can download the ASA tool to look up parts for your montero on mitsubishilinks.com I believe. Thats if you want an OEM woodruff key. A dealer could probably get you the part number too.

[EDIT] Maybe try starting it up and check the engine side of your crank pulley for signs of wobbling with a flashlight. If you haven't already. Could be less things to play tricks on your eyes that way. [/EDIT]