Question for the 383 people.... did the inlet to the water pump move like on the SBs at some time?
The way to "Calibrate" your dash temp guage is to measure the temp at the base of the temp sender and then look at the temp guage. My stock 67 dart with 273 2bbl on cool / cold days the needle sits just past the low end of the normal scale, on hotter days it sits a couple needle widths past that, and on really HOT days it will sit at about streight up.
I have measured the temp sender in all settings and low end of normal range is in the 180 range
a few needles width to the right of that 195
and straight up in the 205 range
all nothing to worry about.
the 205 might be normal for what ever thermostat is in it, 90 deg day will shift the running temp up a bit
I also have a thermal imaging camera and have done the same as you did. What I noticed is the upper rad hose never got 100% engine temp parts of it were cooler. I suspect the coolant flow was not hitting the hose uniformly or the fan was cooling the hose somewhat.
I keep an inch air gap in my radiator so no need for an overflow but with the cap off I see similar to what you describe.
There will not be any flow in the rad till the thermostat opens enough to allow flow through the rad. all the needed flow goes through the BY-Pass hose till then.
Your thermal image of the back side of the rad MIGHT show that there are clogged tubes OR its just the air flow from the fan cooling the rad tubes???
On earlier Pre 70 SBs the inlet and outlet are on the same side (Drivers) later they moved the outlet (bottom) from the rad to the Pass side.
The earlier rads have a baffle inside the top tank to direct the inlet flow toward the passenger side of the Rad
having the inlet and outlet on opposite sides is most likely more efficient but the 67-69 GTS 383s ran that way with no issues.
as for 26 vs 22 inch rad. UNLESS your core support opening has been cut out to match the 26" rad you are not gaining much n the way of cooling as the extra 4 inches are blocked by the core support.
US Radiator has some good info about cores and rows and AL vs Copper Brass
3rd option is to rod out the existing rad. I had that done to a HP/AC 67 Dart Radiator and it was in the 4-500 range.
in earlier (67 - 69) models the V8 shroud was the same for 273-383 (maybe even 440) slant 6 was the odd duck
Your thermal image of the back side of the rad MIGHT show that there are clogged tubes OR its just the air flow from the fan cooling the rad tubes??? if you block the fromt of the radiator with card board you should see a more uniform thermal image if the fan is causing some localized cooling
you don't know what temp it was running, I suspect it was at or near thermostat temp. if it was cool out.
NOW for CONTRIVERSIAL conversation...
If you use a 160 thermostat (which the FSM specifically states is not a good idea) then your engine is not getting up to design temp for bearing clearances etc.
Copper Brass is my preferred material of choice (search "my engine overheats" and see how many start with "I just installed an Aluminum whiz bang radiator" and how many start with "I just installed a stock copper brass radiator")
A factory style shroud will help at low speeds and idle but does nothing at road speeds. If you have a shroud that is parallel to the rad core it will restrict air flow at speed, the closer to the core the more it will restrict.
I have nothing against electric fans IF they are designed for the application. MOST of the electric fans are on shrouds that block 25 to 50 percent of the core area. Do the math, measure the area of the openings and the area of the Rad see for yourself
Thermostatic mechanical fan clutches DO NOT increase cooling efficiency. They only allow for the use of an aggressive fan pitch and reduce some noise when the rad is cool (depending on clutch design and settings) (so technically having one allows for some cooling efficiency when married to the correct fan)
Most likely you could use a 67-69 Dart 383 cooling setup as your starting point, from the water pump to the thermostat, pulleys, fan shroud etc. I would look to the CAP / AC / HD cooling / Trailer Towing options for parts.
Good luck to you.