The "rod" is part of the non-LSD-differential. So you can't fit the "rod" in a LSD differential.
The gear ratios of all the 5-speed Mk4 boxes are the same, the final drive is different. The XR3i has a final drive of 4.27:1 and the RST 3.82:1
IMO stick with the final drive that is meant for the engine. Ford has done a good job in finding the best here.......
