I installed a ally 3 core radiator when I did the conversion its about 55mm thick, but I installed the remote oil cooler to ease the load on the cooling system , my line of thinking was if the oil is at 116 deg and it gets to the oil cooler in the block, the oil will give heat to the water as long as the water system pressure is below 11 psi ( by saturated steam tables steam and water at 11psi has a temperature of 116 deg , at 13 psi the temp is 118 deg )  so to ease the load on the cooling system I cooled the oil by a remote filter which then should either take heat out of the water when the oil gets back to the block oil cooler or both are at the same temperature so there is no transfer either way   
 a bloke at work had a Prado stock std with a 1KZ-TE  and he had a shack up the central highlands of Tasmania, which are approx 1000m above sea level  and he went to the shack towing a 6 x 4 box trailer about half full  and some of the road is quite steep ie 3rd gear with the trailer on, and he cracked the head, then let it cool down and got it to run and drove for another 20km  and then it died and wouldn't start, so he got it fixed with a new head and that lasted 12 months and exactly the same thing happened again so he fixed it again then traded it on a FJ Crusier