Hi again mate,
His reasoning is I think, that if the bottom of the cooling read springs a leak/rots out, coolant will mix with A/T fluid - not good.
I haven't heard of it being a particular problem with the 120, but has happened with a few different series, and in reality, it could happen to anything that has the same arrangement.
This poses the original question of where will it be sited ?
Thinking out loud - replace/have made, new coolant radiator with one of same coolant capacity (minus the oil cooler) and squeeze a separate A/T cooler underneath ?
Coolant hose position should be the same, and seperate A/T cooler pipework shouldnt require too much fettling, as in the general area of original.
If the original dual rad is suspect, I would just replace like for like for a cheaper solution, they go on for years, some never require replacing.
Of course, if you need extra A/T cooling capacity for prolonged, heavy towing, you could go the first route and up the capacity of that part - if it's feasible.
If it aint broke, fix it anyway.