Shayne
There is a specific problem with the auto, the ATF goes through a heat coil which is inside the bottom of the rad - the coil rots and transmission fluid is forced in with the greater pressure, this mixes with the atf, the resultant milkshake rots the clutches in the auto box.
Fitting an external cooler inline is a good idea if towing, but it won't prevent the coil rotting in the rad, so a new rad every, say, 10 years is a good precauition.
A replacement rad is about £110 but if you have a manual get a rad specific to the manual, it will have a deeper cooling area, the auto loses a bit as it has the ATF coil at the bottom. Many pattern rads are generic for both, they have the coil and a couple of plugs for it.
Cheers
Pete