In my opinion, you should drop Visual Arts. It's extremely difficult to get Band 6 in, and when you do, it's scaled horribly because LOTS of people get it. It can also be very expensive depending on your final product. Modern, although may be boring, scales quite good at the 90% area. It depends how you view the HSC. If you see it as a vessel to get you into Uni, then do what'll get you the marks. If you want to enjoy it more than get a 99.95 ATAR, then choose what you will enjoy.
History Extension is specifically about historiography. That is, it's purely about how history is produced and how historians go about developing their interpretations of history. There are different topics that are done in it. There's JFK, Queen Elizabeth, The History Wars (Australia) which show you how historians compile history and have changing interpretations. There are also a few very famous historians (in the field of history) looked at closer to give you a better idea of what historiography is about.
The main idea of Extension History is to get you to create and perform your own research task based on historiography. For example, my research task is about Joseph Stalin and how historians have portrayed him. It's very rarely a good portrayal. The only issue with this assessment is that you CANNOT do anything that crosses over into any (or possibly Senior) syllabus of any history subject. There are certain methods you can take around that to make it acceptable, which your teacher(s) can help you with.
Do what you can during your Preliminary year and see how you go with everything. Reassess what you want to do at the end of your year.
Hope that helped.