Perhaps you might want to find out if the stub axles are the correct type fitted to the front, as I know people who put Torana ones in who struggled with handling because I think the castor or camber may go all up the shit if you use those and in Adelaide they often do very bodgy work (people because they don't get inspected each time a car is sold) and don't know this. It is most likely worn parts or a castor issue if it is floating all over the road at higher speed. Does it scrub tyres? I don't think it would have much to do with the actual body structure itself, unless it was really bad. A good way to start is to get your mechanic to identify what brake calipers you have on the front and what diff you are running at the rear and let us all know the result.
or post up some pics if you don't have the resource of a mechanic and I am sure some of us could let you know what you have got.
