66 Coupe overheating problems

I have the same motor you have, and had the same problem. I found my fan was too far away from the radiator to be effective. This was caused by the previous owner installing a flex fan without using the correct spacer. You want your fan to be about an inch or inch and one half from the radiator. You also should have a fan shroud as this will help the fan suck the air thru. Use a 180 Thermostat, make sure your fan belts are tight, and yes, a 3 row radiator is a must.

It's VERY simple to cure your problem.

1) are you running a radiator shroud? I bet you're not. You have too if you don't want to overheat at slow speeds. The reason is that your fan is pulling air around the radiator rather than through it. The shroud forces air through the radiator Actually, placing the fan really close to the radiator will have a similar effect; but, that always makes me a little nervous.
2) even if you're not running a shroud, when idling, your engine is only turning like 600 rpm. Your fan simply isn't turning fast enough. My 351 has no shroud and it gets hot at lights. I just pop into neutral and rev the engine to 1500-2000 rpm. It's a little annoying; but, you can watch the temp start dropping immediately. I'll be adding a shroud soon.
3) If that doesn't work (or if you already have a full shroud), I'd look to spark timing. If it's too late, things can get a little hot.

I hate to act like it's so simple; but, you said your pump and thermostat are fine. Also, you've got a new aluminum radiator. Based on these facts, I think #1 above is most likely your trouble.

Lst1standing;
I had the same problem with my 68 302 engine. overheat at stop lights and going up hills. I had the stock rad. with a fan shoud plus air condition. I replaced the stock rad. with a Modine 3 core from NPD and that cured the problem.
68 dailydriver

Get a 3 core radiator, a fan shroud, and either a stock fan, or a flex-lite 17" fan. Be sure the fan blades are about halfway into the shroud opening. Use 50/50 mix antifreeze/water. Make sure you have a 180 degree theromstat.

Most importantly.... check your lower radiator hose to be sure it has an internal support ring. A very common cause of overheating is the collapse of the lower hose, at operating temperatures.

You can also use an additive like water-wetter. It will reduce your temps by about 20% or so... its an excellant product for $8.95. You might also want to consider an electric fan, as they are much more efficient, but also a bit pricey.