I'm looking at a 1970 Dart FSM which is less than helpful, but one thing it is clear about is that the key has to be lock to get it back in
" (2) Insert the lock and key into the latch through the glove box face.
(3) Hold cylinder in position and remove key."
Ok I just spent a half hour with three glove box locks - orient inner cylinder so that the parts that are sprung toward the outside of the shaft to the left as you would see it sitting in the car. If you push the key in all the way all the bits will go in except the one closest to the knob - this is your problem. pull the key out, push the bit nearest the knob in with your finger and then push the key back in - all the bits should stay in. Push the inner shaft into the outer shaft and pull the key out. That one bit should spring out and hold the inner shaft in the outer shaft.
What I can't figure out is how to get that bit pushed in while it is in the outer shaft!!! How did you get it apart to start with???