I am running a s13 welded diff on my s14 right now. IT BOLTS UP~!!!!!! dont worry. Main thing that you need to change is the diff cover. The s13 diff cover has an open atmosphere breather and the S14 diff cover has a nipple breather that you put a hose onto. (it's already on the car, you'll see it when you swap em out). Ok, so the main difference between the two pumpkins (besides the cover) is that the s14 diff has large holes and rubber spacers to go in there. To use the s13 diff, take the washer looking rubber piece from your s14...not the T shaped piece....and simply put that between your s13 diff and your s14 subframe. The two bolts that you attach the diff to the chassis have large washers and will secure the diff perfectly. So ignore what the shop said, and just do it yourself. It will only take about an hour if you know what your doing. If you have any questions, PM me.
GOOD LUCK
