The stock radiator should do just fine, that is if everything is clean on the inside of the radiator, engine block/heads, water pump and the thermostat is opening all the way when it's supposed to.
This is a '66 352 and it would idle all day long and not overheat just like this. In fact it's the original '66 radiator as well as the heater core and everything is connected sans the dealer installed air con at this point and it all works just fine. There is no restrictor for the heater core. This is 100% stock setup in the picture, even running on points still. I didn't even install the fan shroud and yuppers it does come with one but it's the limp little direct drive 5 bladed fan keeping everything cool and the vacuum advance is even connected to ported vac, if connected to manifold vac it would idle even cooler.
Why this never overheats purring away at idle is because I cleaned out the block and heads by totally disassembling and cooking them for several hours at 400-425˚F. You would not believe the crap (now dry dust/dirt/scuz) that blew out of the cooling passages. The water pump was just replaced with a new one and for the radiator, whilst it looked OK from peeking into the radiator cap, I bought a few gallons of distilled vinegar and heated it up to boiling, laid the radiator flat with the cap on with the ports sticking up and filled it up with the very hot vinegar. It was a little reactive volcano as all the minerals were dissolving rapidly in the mild acid (vinegar = acetic acid). And after 15 or so minutes all the crud that came out was amazing for something that looked clean to begin with.
Had I not cleaned out decades of crud from tap water and old coolant and other contaminants this engine would probably suffer a similar fate to yours sitting at idle with a propensity to overheat.
Oh I should mention, if everything is clean and working properly, you do not need an overflow tank as its technically built into the top tank of the radiator, you are not supposed to fill these radiators to the brim. But at about halfway on full on the top tank. I didn't even get round to attaching an overflow tube (in the event it should overheat) but even with a brand new pressurized cap, not one drop of coolant ever peaked out.
Now if you want to look into a better radiator than the stock one, especially if you have air con or a high horsepower engine you might want to look at this thread. I recently bought a heavy duty 3 row high density (more columns) all new brass/copper radiator for the '66. But there are pictures for comparison against an original '66 and the new one if you would like to have a look.
Also if you do have air con (factory or dealer/aftermarket) you might want to consider replacing the direct drive fan with the 7 blade thermostatic coupled fan. When the clutch fully engages the fan sounds like a bloody industrial leaf sucker truck from the Midwest. It moves an amazing amount of air when fully engaged and just limps around when no cooling is required. And of course it operates in between those two extremes as well.
Some thoughts and considerations to ponder.