It's NOT that great.
Cars come with differentials for a reason...and a welded diff sucks IMO.
I ran one in my S13 for a short while (had a spare diff and an open afternoon...so I grabbed a spare open diff from the shed, fired up the welder, and put it in...
First thin you'll notice...you can hardly turn while backing up...it just goes straight, and drags the front tires.
Then, unless you're going for big power oversteer, it understeers badly since both rear wheels are trying to go in one direction, and it just causes the front end to push.
In short...just a buy a VLSD.
You won't feel as "hardcore" or cool, but it's a better solution, unless this is a track only car.
http://www.ka24development.com/vlsd.html
- Brian