It sounds like your fork isn't engaged in the bellhousing.
![Image](http://i280.photobucket.com/albums/kk191/nosale5000/Ford%20F100/IMGP3440_zpsz0xdctmy.jpg)
Your clutch fork should have a clip that engages that slot you can see inside the bellhousing.
![Image](http://i280.photobucket.com/albums/kk191/nosale5000/Ford%20F100/IMGP3442_zpszrnbjop8.jpg)
This shows the clip on the back of the fork.
![Image](http://i280.photobucket.com/albums/kk191/nosale5000/Ford%20F100/IMGP3444_zpsdb9esxec.jpg)
The tines of the fork engage the bearing like this.
![Image](http://i280.photobucket.com/albums/kk191/nosale5000/Ford%20F100/IMGP3446_zpsmuebhkjg.jpg)
Here you can see how the clip on the fork and the clip on the bearing work against each other to keep themselves engaged. The fork shouldn't be able to slip off the bearing, I wonder if your fork has a clip on the back.