The rules state that only units with the Burst ability can use an Ammo Belt. Must they be using Burst fire in order to use the belt in this case (Meaning that all units must be within 2 spaces)? Or can the fire be concentrated on one opponent unit with the aid of the belt and beyond 2 spaces?
The requirement for a unit with Burst ability is because those are the ones that use belt-fed weapons -- the machine guns. There is no requirement that the unit be performing a Burst action. DPG was just tying the bonus thematically to units with belt-fed weapons.
I am a bit confused by your last question, but it leads me to wonder if you understand Burst fire correctly. All the target units do not have to be within 2 inches of the shooting unit, they just have to be within two inches of each other.