That's easy. If your factory harness is in OK condition, you can just plug the new ballast in in place of your old one. Leave the factory wire hooked to the coil + terminal. Now all you need is to pick off power to go to the box. If you have trouble identifying which side of the ballast to hook to, UNPLUG the ballast, and turn the key to the "run" position, then see which wire which goes to the ballast has power.
That is the (blue) wire that in your new diagram goes off to the box.
Is this a /6 or v8?