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This started off as a radiator core question and look were it's gone.
This seems to be a common issue because people believe thermostat "controls" engine temperature to the temperature on the package. I've posted on this subject before, it doesn't work that way, folks! Also, lower wall temperature affects combustion and can causes other issues; power, lube, sooting, tolerances, emissions, etc.
Actually, higher coolant temp will dump more heat out the radiator, so it's counter intuitive as well. {It's an easy calculation if you know heat transfer.}

Where has a discussion about a core question asking for dimensions and specifications gone? To a contributed description of a core with dimensions and specifications that works well for a Packard Straight 8 engine as one person's example. I don't see where anyone is saying thermostats control engine heat, but they can affect it, if they don't open when they should or, if, as Joe pointed out with the 140 and 160 thermostats, they restrict coolant flow. I find that facts, not officious comments, serve the solution here. This thread is one of the more technical ones. But even God knows, Jim, that it takes many efforts in different ways from different perspectives to communicate if one wants to get through to us pea brains.

Well, if I hadn't gone out and taken picture for this one, I might agree, Joe.
No good deed goes unpunished as they say. I'll be silent from now on.

Don't be silent on my account, Jim. You contribute a lot of good advice, but you characterized my post in such a disparaging way, I couldn't let that "You've got it wrong, I know it all, and I'm tired of having to repeat myself" attitude pass. Maintaining patience at 75 isn't always that easy, but I guess I draw the line at impatience and ridicule in what, for most of us, is a learning environment. Please don't curb your experience. It has value.

I realize that this is an older post but I want to work to complete the story. After many discussions with experienced radiator people, I am coming to the conclusion that the differences in the radiator are not as significant as I thought and that since the problem mostly seems to be at idle, better air flow at idle seems like it will help.
So I have decided to go with a larger fan and then retrofit a shroud to capture as much of the radiator into the airstream at idle as I can. I will report back once I have the fan and shroud fitted
I am going to try to have the local machine shop modify a 6 blade fan shown here in the picture by opening up the 5/8 inch center hole to fit on the water pump hub (which is 1 ? inches).
We cannot use an adaptor because the fan cannot be moved any closer to the radiator so it seems one solution is to try to have the fan mount directly on the existing hub.

Here is one very good comment from one of the experienced radiator experts on differences between the original radiator and the new one:

I too would have made the radiator in the flat fin design with a larger (more elliptical tube) in a staggered pattern. I would have suggested a fin density to 10 or even 11 fins per vertical inch.
PS: Measuring 11fpi in a serpentine core is not quite the same as 10 or 11 fpi in a flat tube and fin. Flat tube and fin core have holes that are punched and then flared. The flaring provides substantially more contact area between the tube wall and the fin that the serpentine where contact is the radius at the apex of the v shape. The serpentine has far less labor involved in the construction which is why it is so prevalent. It also has a shorter life as fins can separate from the v vs the flat tube where the fin is "press fit" into the hole.

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I've had the same sort of problem. I'm told that any of the radiators are big enough to cool the engine under all conditions so I copper recored mine. While mine was out, I took off the water pump, pulled out the water dist tube, and stuck a pressure washer into the hole. I even made an extension for the upper rad hose and, after removing the thermo, blocked off the lower exit hole and ran water backwards thru the head. After about 30 gallons it was running clean, and the stuff that came out of the block looked like life on Mars. Runs pretty cool now. Also, I suggest getting an infrared temp reader gun off of ebay so you can read the temp at different places on the block and radiator.

Thank you.

I have had the engine rebuilt, new water distribution tube, new radiator, new water pump, new (and correct) 160 deg thermostat, new (and correct) exhaust system, distributor tested and fixed, vacuum advance fixed, etc.

So I should be very close to solving it.

Joe, regarding fitting the after market fan. You may be able to use some thick flat washers, thick enough to get the fan even with the end of the water pump shaft, to mount the fan. The fan shaped slots for the bolts will center the fan. This worked for me. The fan was aligned straight and rotated smoothly. It looks to me that if you bore the center hole large enough to fit over the shaft, it will cut into the slotted holes. JWL

JW,

You are right. I think I may have found an older Packard 5 blade fan from an early 50's car that had A/C. The supplier told me that the bolt hole spacing is correct but the hub center hole is smaller than what I need. So I am going to try to have the machine shop open up the center hole in that fan and it should not hit the bolt holes.
The machine shop claims that they can open up the center hole and not lose the balance of the fan.

Joe, have you looked for a Packard 5-blade fan? They are out there and easier - and probably cheaper - than modifying some other type. JWL