2001 Montero Sport 3.0L
OEM muffler measurments, using a measuring tape:
Pipe Diameter 2"
With these measurments, I ended up ordering a Magnaflow, part # 12244. This has a 2" piping. You can go with part # 12245 if you want 2.25". The Magnaflow I ordered is 5x8 oval 2" diameter pipe, 18" body, total 24" including pipe extrustions from front and back of muffler and satin steel. I figured I'd save some money by not going polished.
I'll just find a nice chrome tip and make a side exit exhaust.
I've talked to several muffler shops and it seems Magnaflow is the recommended choice with Flowmaster coming in second.