I think this article will help. It's based on my research and on my own struggles with spey casting.
Getting Started With Spey Casting
Quickly though, I guess you first have to decide what kind of line you want to cast. With a rod of about 12'6" I'd go with either a Skagit, Rage or Scandi line.
Rods used to cast Scandi lines tend to be tip flex. Rods used to cast Skagit rods tend to me more full flex.
If you're planning on casting a lot of sinking lines and/or heavy flies, I'd go with a Skagit. If you're planning on casting smaller flies mostly on the surface, I'd go with a Scandi. If you want a hybrid of both lines I'd go with a Rage.
Finally, with a 12"6' spey rod I personally would not go with a longer-belly line.