Thanks all for your constructive comments. Yes it is a big job and I wanted to work out what is the best way to go about it. The diagram from HK1837 is particularly useful and gives a great insight to how Holden put these together. Maybe it is better to try to fix the WB but I had my heart set on the HX.
Here is a couple of pictures of the HX. It is a factory 308 4 speed car with integrated air. It would have been fantastic when brand new (I hope the pictures work):
DSC_0446a.jpg
DSC_0535a.jpg

The WB has been bent on the rear LH quarter panel and I think I'm more afraid of fixing this than swapping the roof sections. As you can see the quarter has squashed in the panel so it is not square with the tailgate:
DSC_0735a.jpg
DSC_0737a.jpg

I would appreciate your comments. Thanks.