i've sourced a heavy duty radiator for my 18RC turbo.
it uses the same tanks off a standard 18R radiator but gives you a 3-row core - much better suited to turbo applications
standard ADRAD code: TOY9182
cost me $330 plus $35 for the tank-donor radiator from a wrecker as i didn't want to disable the car while this was happening.
any decent radiator shop should be able to source the core, all they do is stretch the tanks over the core and seal it.
Also, if you're doing the 1G install and are looking for a radiator to (just) fit you RA40 - then a holden unit can be used, tho you have to sacrifice the aircon dryer/collector or shift battery from under-bonnet. as it's wider than a standard RA40 radiator
code: HOL9114
it has side tanks and you have to attach a remote filler, plus you gotta 'burp' the system after refills as the connection point forthe remote filler is halfway down tank.
There is another option on the radiators in a ra40. If you wanna use a holden commodore radaitor out of say a vh or so. You can get the core cut down to wateva size you need. All they do is cut one end off to where u need it and refit the header plate. Does cost a bit extra but its simple enough. Thats what im doing for my ta23 as it is alot safer then spreading tanks to fit a 3row. i know this as i work in a radiator/intercooler/air conditioning shop.