I did a "How-to" post on the brake upgrade for a 2.6l SWB. If you replace the rotors, which I would recommend, most everything will be the same.

Montero Gen 1 brake upgrade

As far as turning in a single piston caliper - FWIW, they never even looked in the box when I returned mine. I half expected to be charged later, but never was. I turned in my single piston 2.6l SWB caliper, which isn't even close to what I bought. <img src="/forums/images/graemlins/smile.gif" alt="" />

On the flip side, you can find cheap calipers with brackets on Amazon and RockAuto with no core return.