I fitted a Modine oil cooler to my 2.9 twin turbo when I built it. Iol temperature was fine but the coolant temperature was too high. I replaced the Modine with a thermostatic sandwich plate and Mocal 13 row cooler. Oil temp is still fine and the coolant temp is now back to normal. The cooling system couldn't cope with the extra heat from the oil as well. Fitting an oil cooler will reduce the heat being dumped into the cooling system so it may be your best bet.