SI and ITR have similar gearing (Thought to be the same but there are some lingering questions about Final Drives). It has been found that some SI trannys (4.2) have a different final drive than an ITR (4.4) tranny. I'd just buy a Type-R Tranny and call it done. YOu coudl change the FD drive too but that woudl put you in shock going from an LS tranny. My car has a 4.929 FD and I rev 4800 at 80 mph in 5th.