I tried many many different things with mine, as the 4.2's like to get hot under load
I found the Factory cooling system "just" adequate for what it has to do
but, after many many dollars, I found that the Factory setup worked the best
my initial advice
flush the motor and Radiator
fit factory thermostat (but check it first with thermometer & hot water to make sure it opens when it should, and at what temp it is completely open)
check & replace Viscous hub & Fan
flush & or replace Rad with new one - factory
then get yourself some gauges, so you know what the engine is doing
I have had 3 different types of gauges in my car - first 2 types didnt quite do what I wanted
until I spent B I G $$$$$ of what I have now
3 gauges that are 3 gauges in one
Boost / EGT's / Cabin temp
Oil Pressure / Water Temp / under bonnet temp
Battery 1 / Battery 2 / Amp draw