The 250 2V head or the Argie heads are just a head that mount on your block, exactly like the current head you've got on now.
The difference is that if you look at your current head you'll see your carburator mounted on an intake that is cast as one integral piece with the rest of the head.This definitely restricts breathing. (air and fuel mixture coming into the engine).
The Aussie or Argie heads come with an intake manifold that is bolted on. These intake manifolds are much better designed.This allows you to use different carburators, allows better breathing etc.
As far as the headers.. if you install the 250 2V (aussie) head you can use the same header as you would on your current head. The Argie heads need a different type of header.
Whether you end up with single or dual exhaust depends strictly on the type of header you buy.
Rick