I haven't found any reference to a jet valve in any of the 3.0L engines, but obviously from this example, they do exist.

The reason I'm cautious about using heads mixed from separate engines without checking head thickness and combustion chamber volume is the possibility of ending up with a different compression ratio on each side of the engine, which may produce a better burn and changed gas composition on one side as compared to the other.

The ECU trims the fuel mixture by (among other inputs) reading from the single o2 sensor, which is behind the Y-pipe collector where the gases from both sides merge. The sensor will read an average of the combined gas and will fuel both sides of the engine equally. There is a possibility that the resulting injector pulse width could be too short for one side of the engine or the other, resulting in a rich mixture on one side of the engine and a lean burn and pre-ignition or even detonation on the other side.

OTOH, that worry is really a maybe and might not be a concern in practice, particularly with the conservative ignition curve built into the Gen1 Montero ECU.

It's worth a try to save some money..... just be sure the correct cam is installed.

Frank