there's your problem. Tighten that bad boy up for a bit until around an 1/8th inch of thread shows, then give it another whirl. There's a hole in the end of the shaft that comes out the other side of the Tranny, stick a small screw drive in there to hold the slipper assembly in place while you tighten the 8mm nut. Make sure when you put your motor back in, you check your mesh between the SG and the CB. You can adjust this with the two long bolts on the each side of the motor mount. Crack the back one and then the front. Make your adjustment with the front and then tighten the front back down, then tighten the back down. (Note, front and back are based on you looking at the car from the side with the motor closest to you).
Give me a holler if you need more help.
Cheers,
Curt