Unless you are going to put original antennas back on just throw the broken piece away. The stud you bought should mount right on the tube coming from under the trunk mount. You will need to buy a short pl295 to Motorola cable for the CB side to go from the box under the trunk to the stud, the antenna should screw right into the stud. For the AM/FM side you will need to cut the cable and put a new PL295 connector on it or buy a pre-made one and run all the way from the front. A CB antenna will work for the AM/FM but not optimally DO NOT use an AM/FM antenna for the CB! If you need more help give me a shout after 1800.
I am no sure if the Harley antennas will work at all since Harleys have a load coil in the tour pack for the CB antenna, so when you mount them make sure you run a a VSWR check. Stock Harley antennas also have CB stamped on the side at the screw end or at least mine did.
Cable
http://www.walcottradio.com/18smmpl-pl259-to-motorola-plug-18-long-p-934.html
Stud
http://www.walcottradio.com/firestik-k4a-stud-mount-pl259-screwon-connection-p-247.html