Blizzard
Once you've hit the required Conquest Points you'll receive a quest where you can choose which weapon you want (based on the weapons your class can use), and the weapon will be of item level 345.
The 280 item level on the Conquest bar is a display error which should be fixed in an upcoming update.Blizzard
Just wanted to let you guys know this is actually a known issue and we're currently working on a fix. Once we get that fix applied, which should be soon, you should receive the proper new achievements and appearance rewards.