It's not as simple of cutting the mounts of swapping them over. I looked into doing the swap on a mates GFs coily about 18months ago and converting to a 3-link with a jimny coil/shock setup. The factory coily front end is a rubbish design and it very limited in ways of getting more travel and improving the ride quality (as well as been weak busting mounts etc..)
If your going to the effort of swapping the diff out, it would be a good opportunity so ditch the silly coil over setup and swap to jimny coil/shock bucket. You may have to move panhard rod mount on the chassis to due to the fact the WT diff runs the tie in rod in front of the diff unlike a coily which is behind the diff. It gets tight but it possible with out bendy panhards/notched out sumps. So in short, it would be better to start from scratch if you want to gain the full benefit of doing the swap (providing you have the fab skills to do so). The only thing you would really use is the radius arm mounts, even them I would be faster to fab new ones by the time you cut the old ones off and cleaned them up.
As for width, how much difference is there? 10-15mm? Me personally, I would worry about, but thats just me.
Have a read of this thread here :
http://auszookers.com/forum/viewtopic.p ... shos+coily