Do you have an
oil cooler as well? Preferrably a big one.
Make sure that the air has to go through the radiator (ie that the gaps around the radiator are closed up, and that the fan has a shroud.
What kind of radiator do you use - one of the big aluminium ones?
I have an aluminium radiator, large
oil cooler, run standard 50:50 mix of glycol and water, have one electric push fan, an electric water pump - and I have problems getting my car warm enough!
Actually - writing all of that - what is your water pump like - maybe not circulating enough?