Mitsubishi engines are measured and spec'd in metric terms, so .039" = 1.0mm. Not that you can't convert to inches. The FSM makes this a little easier, since it gives specs in mm and inch measurements.

Standard overbores (rounded off conversions) for the 3.0L engine are:

.25mm = .010"
.50mm = .020"
.75mm = .030"
1.0mm = .039"

Any overbore to the next size (or greater) will require new pistons.

You'll get a slight CR increase as a result of an overbore.

The service limit is .008" milled from deck and head combined. You can go more, but your cam timing will change and you may turn a non-interference engine into an interference engine. You can reset the cam timing by installing eccentric bushings in the cam sprockets.

Another issue with severe decking is that the pistons protrude ~.015" above the OE deck at TDC. If you take too much off the deck, the piston-to-deck clearance/quench height with a .050" compressed height gaskets becomes less than .035" - not a bad thing on an engine that turns 6000rpms max since it helps with detonation resistance.... but I probably wouldn't want to go below .030".

You can change to SOHC Diamante pistons and get 10:1 static CR, but you'll have an interference engine that's running on an 8.9:1 ECU which with the Montero cams will yield a Dynamic CR possibly too high to survive. The answer is to swap to the 7? retarded SOHC Diamante cams and install them with no advance. I don't know how that combination will perform - maybe ok, maybe not so good on the Montero ECU. Your torque peak will be 500rpms later and higher.... and perhaps not so well matched to the truck gearing - particularly with large tires.

There's not much to gain from the SOHC 3.0L without serious work, but a fresh stock 3.0L is a nice engine with adequate power.

Oh....... I don't know how MMC got 8.9:1 as the static CR. I haven't found it to be that high. Perhaps they use a number that reflects the maximum rework to service limits on decking and overbore to arrive at that number.

Good luck,
Frank