The actual back panel doesn't go around the corners so you don't have to worry about the different side profile on the lamps, and the one I got didn't have holes for the numberplate panel, so you won't have to weld them up.

The bolt holes for the lights are definitely different but I can't recall if the holes where the bulbholders go through are different - I think they are, but nothing you can't work around if you really need to.

The chances of finding a good Mk 1 to chop it out of (and not damaging the panel in the process!) are pretty slim, but it might be worth a trip down to a scrappy with a Mk 1 tail-light and try to put it into a Mk 2 - just make sure they know it is your lamp so you don't have to pay for it on the way out!!