I love the idea of using MS on the 3.5 - with the tunability you'll have I think you'll be able to get significantly more power out of it than stock, especially if you have better-flowing intake and exhaust systems. The 3.5 is factory tuned to run on 87 octane but will advance the timing to take advantage of 91 if you use it. Tuning yours specifically for 91 octane would likely enable even more power and torque.

Please start a thread documenting your MS installation.

95 Montero SR
3.8 MIVEC, Advance headers, 2 1/2" exhaust, Magnaflow muffler, OME shocks & rear springs, 2" body lift, 3" tank lift, 4.90s, TRE front locker, factory rear locker, Roger Brown Rock Sliderz, 315/75r16 (35") tires, Sport big brakes