Welded diffs are fine once you are moving along, they are a bit of PITA at low speeds though.

Thing is though, the will exagerate a lack of front end grip, and most 340's have fucked racks and knackered bushes and really poor front end grip, so the rear gets even more say in where the front goes, if you weld up an MX5 though by comparison it drives surprisingly well still even at really low speeds.

So its as much cause the car is shit as cuse of the welder IMHO