Disco, since you're going to cut the nut off, why not order the new one first? That way you can see how thick the aluminium is and be a little more confident that you don't go too deep.
Do you folks think it would be possible to split the nut, once cut, by wedging the two fins on either side of the cut apart? Might break the fins, but that won't really make things worse and I reckon it's worth a shot before taking a chisel near the head.
You definitely won't saw too deep too fast as the butterfly nuts are pretty hard, as far as aluminum goes. You'll know when you're there as the coarse threads do not go all the way into the b'fly nut threads; ie: you'll see 'em before you saw 'em.
On my particular experience, wedging the chisel into the cut in a horizontal position spread the nut enough for easy removal without the need to break the part into two pieces. "Hammer and Chisel" sounds rather violent but be gentle. It's aluminum and you won't need much of a whack.
You're not busting rivets off an old iron bridge.
