See section II of the attached manual. The sun gear is attached to the lock-out rod by a slot in the sun gear. If the part that connects the sun gear to the lock out rod isn't in the slot on the sun gear, the lock out rod can't move the sun gear to lock the transmission for reverse. Also, make sure the side cover you put on the transmission has a tab on the reverse shift arm to engage reverse lockout.
http://www.oldwillysforum.com/forum/...riveManual.pdf