Agree, but that's also why I was asking about what else was done at the same time. The most obvious thing is the condenser changing the air flow, but was the radiator changed or relocated to accommodate this? Were the coolant hoses changed in any way? If the water pump came with the crate engine, then we can at least assume for now that it's the correct orientation and in reasonably good condition. I hate assuming of course, but just to narrow the search down a bit at first...
The fan would still be a good place to check, even though it's not given you trouble until now. Most of the time when a fan is on backwards even a stock engine with no A/C will not cool properly. So it's usually discovered long before any additions come along.
But a weaker fan might have been borderline before the change, but now just can't keep up with the demand of pulling through two radiators.
How old is the radiator I think someone asked? Even new engines can puke casting sand and other junk into the cooling system, and things like that tend to coagulate at the bottom of the radiator and block flow through several layers of tubes. A stock size radiator is more than enough for a mild engine, but a half-clogged one won't be.
The air dam is an almost universally good idea, but especially when you have trouble with air flow. It's not a cure-all for other issues, but you are at least eliminating one layer of compromise. And adding a nice tool tray at the same time doesn't hurt!
http://www.wildhorses4x4.com/category/s?keyword=air+dam
Also mentioned was removing or louvering or meshing your inner wheel wells. This is also a great idea.
However, both of those air-flow improvements benefit an EB at higher speeds and low. Especially with a weaker fan. They still improve low-speed air flow, but not as much as at higher speeds. In theory at least. Lots of owners will tell you that their Broncos stopped overheating when they removed the hood and put it on the roof! Air flow.
Some have benefited the same from the mesh at low speeds, but not every overheating issue is fixed by doing that either. And you don't seem to have an issue at higher speeds as you've said it cools down quickly once you start moving again.
Have you also attempted to simply rev the engine up while sitting still and overheating? If that works, you could still have an air flow issue, but could also be a coolant flow issue.
I know... Overheating is such a bitch!
But a tuneup is paramount. You've been driving it for years without one, and perhaps it doesn't need it due to electronic ignition and a well-tuned carburetor. But you never know. You could have been on the borderline of running hot before, due to a poor ignition timing setting for example, and putting in the A/C simply put it over the edge.
I am now running a 5.0 with A/C, Explorer mechanical fan and stock radiator. On cool days it rarely goes over 185. On warmer days it goes right to 195 and pretty much stays there except under special circumstances. When it goes up maybe 5 more degrees at the most. Haven't been running it for all that long yet, but so far that's what I've seen.
Oh and it's also got a winch on the front, which adds one more layer of air disturbance ahead of the radiator.
It might be the fan, but I'd bet a TON of that consistent cooling is due to the finer tuning of the EFI and computer controlling the timing.
If you ever have a chance to watch real-time timing changes through an OBDII scanner, you'd be surprised to see just how much time the ignition stays at 20° or more. Only coming down under heavier use of the accelerator.
So if you do nothing else, a quick timing check and change might at least be worth the experiment time it takes to do it.
And how is your PCV valve? Checked it lately?
In your location it can certainly get warm. But what gas do you get there? Ethanol mixes? I wonder how that's effecting heat output? No idea, but putting it out there in case someone knows the science behind it.
Good luck.
Paul