You can use your stock diff if you use the stock trans. If you do tihis you will have a very fast SRT becuse the n/a has a much lower 1st gear and the 3.27 will help also. I am using the stock n/a trans with my V8 and have had no problems. My V8 has a bit more torque than a SRT. If you do this the swap will be easier and you can swap trans latter if needed in which case you will probably need the diff and SRT TCU.