When using a 1/4" sq drive cable at the engine, what keeps the cable and prop shaft in place? I guess what I am asking is what parts do I need to use outside the back of the boat.

3 ways pending set your using ?

Some times cables get soldered into a brass ferrel that in turn set screw attaches to stub shaft ... This set up is SELF RETAINING.

Next if using the 8mm stub shafts, some will have under turned diameter mid shaft and a short screw comes threw side of strut into this under turned area preventing shaft slip out.

Last is using split shell compression collars around cable at engine end right next to square drive collet.