The diff probably wouldn't last too long. I have a 120y diff in my ute, but i wouldn't expect it to handle much more power (I have a standard 200b motor in the ute, 2 litre) It may work for a while if you take it easy, but if you intend to drag race, definately upgrade. I don't think there is much in the way of straight swaps. You may have to get one cut down.

The transmission requires a bit of chopping to get it into a 1200, i think it be easier to put it into a 120y (I think...) Its not too hard, as long as you have a grinder a welder, some spare sheet metal, and a brain

Here is what i had to change on my car. the shifter hole was moved about 200mm back, and a section was made higher.