A good rule of thumb for exhaust system sizing is to go 1/4" larger than the stock diameter. Couple that with a low-restriction performance muffler and California-legal cat (the 49 state cats are almost all made in China and will only last a few years) and you'll have a system that will significantly outflow the factory system to gain a few HP on the top end but won't sacrifice your low end torque. This is a good place not to cheap out on components. Get it done with good parts and you can forget about it for the next 10 years.

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