You need to take your high quality original metal radiator to a radiator shop and have it repaired/rodded out, then put it back in your Montero to replace the aftermarket garbage that you replaced it with. With few exceptions the aftermarket radiators don't have cooling capacity equal to the original, and they are far less durable.
Next, how do you know it's hotter under the hood now - have you done any temperature measurements?
Hood scoops or vents are an unnecessary bandaid for a cooling system that's not working properly - the original cooling system is capable of dealing with the heat from all but the most strenuous rock crawling, provided it's in good condition and functioning correctly.