And yet, when I got my 1200 coupe, the radiator was in a bad way, so I had it re-cored with a standard replacement.
I also cleaned out the cylinder block, replaced the t/stat, the water pump & the fan belt.
I included a cheesecloth condom type filter in the radiator, which proved a wise move, & have not had a problem since.

Even in the mid day heat of last summer, along the Hume Highway at full highway cruise speed [110k, or 68mph] the temperature actually fell slightly, compared to town driving. & it sat almost dead on the halfway mark on the gauge.

Since the vehicle needs X amount of power to move at that speed, regardless of what engine is producing it, then it would seem reasonable to assume that the same amount of heat would be produced, regardless of the engine type, capacity or form of induction.

If my stock radiator has no problem handling it, then I see no reason why a clean & fully serviceable one wouldn't handle a larger engine that was performing the same highway cruise duties.

Lets get back to the cheesecloth condom.

When my radiator was out for servicing, the crud in the block that I couldn't reach dried out, & like clay, it apparently shrinks & cracks. When the engine is returned to service, these little blocks of crud expand & shrink with the heating & cooling cycles of the engine & some break off.
At higher engine speeds they are carried into the coolant circulation stream & up into the radiator which acts as a filter to ensure that this crud doesn't return to the block.
In other words, a new radiator can be rendered unserviceable within a very short time because of this.

I was amazed at just how much crud was captured in the cheesecloth condom filter & I have washed it out three times now with very little in it the last time I did it.

A partially blocked radiator will handle the low heat output at low speeds, but may not have the reserve capacity to handle the higher heat output at higher speeds.
Have a good look inside the 1200 radiator & see if you can see any crud in the few tubes that are able to be viewed. Reverse flush it if you see anything at all.
No point in wasting a perfectly good radiator if you can clean it out.