Hi coolgoose,
The '78 exhaust bolt holes that go to the frame (near your footpegs) are in a different location. I can't remember the exact length difference, but it is something like 1/2 inch to one full inch difference from the front of the engine.
You would have to a do massive amount of work. Better to stay with what you've got. Or, if you've got the money and really concerned about the looks, do the math and see which is cheaper.... new or re working what you have.
~ ~ ~ jaknight ~ ~ ~