Once again, I am having a problem understanding how none of these radiators worked. You don't have a filled block do you? Something else is going on here. It all boils down to this; either your engine is making too much heat or your cooling system is not working. Look at these 2 issues and approach them methodically to determine what is happening. My feeling is that you have gotten drawn into a sequence of parts replacement hoping to fix it rather than your mechanic understanding what is actually happening and applying an adequate solution.
Given the radiator changes, you should have arrived at one that was adequate by now. That 2 core/row should be moving a lot of fluid, then it comes back to the fan/shroud as to whether they are pulling enough air across the radiator to cool the fluid. Of course the water pump is in the mix for moving the the fluid but unless it is a complete POS I have a hard time believing this is the issue as generally they either work or they don't; and they typically work more than they don't.
Like I said previously, I'd throw a 160 deg T stat in to see if the system can abate the temps better as right now, your engine is producing more heat than your cooling system is capable of reducing. So, in my mind, if the coolant is allowed to flow earlier (at a lower temp) perhaps the cooling system could cool it quicker and manage the temperature rise. I'd also try some Water Wetter to see if that helps, it will allow heat to dissipate quicker.
Before I did all this, I would take a hard look at the timing and see where that is. If the timing is decent and you have addressed the rest and still have overheating issues, you have block and/or head issues meaning there is some restrictions in the system somewhere. This is where your IR thermometer comes in, see if you can locate high heat spots on the engine.
I know this is very frustrating, but if you stay at it you will get it figured out. Doesn't mean you will like what you will find, but you will at least figure out what has to be done.
A friend and I ran into a similar issue with a 440 6 pack motor he bought once (back in the late 70's). It had been pulled from a wrecked car and rebuilt and then sat waiting for a car to be put in. My friend heard about it and was able to make a deal on eventually. We got the motor (which was very clean) and presumably it had all very good internals and was "ready to go". We installed it into a 70 Convertible Challenger and it was a monster in terms of power. Problem was that it would overheat all the time, no matter what radiator, water pump etc. we threw at it, it would overheat. This was before the days of big aluminum radiators and such so we had to use what was available; we tried the biggest radiator we could find/fit, different fan and shroud configurations, even the new fanged "flex" fans, all to no avail. Finally we got fed up with it and pulled the engine figuring there was some defect we just could not locate. Upon pulling the engine and popping out some of the freeze plugs, we discovered that the water passages were almost completely full of dog food. Turns out mice had nested inside the block and brought in dog food as supplies and had done so in very large quantities. It took use 3 days of flushing and fishing around to get it out. Put new freeze plugs back in, reinstalled it and it worked as it was supposed to (in fact ran a bit too cold due to the HP cooling system we had constructed).
I hate to say this but given that this is now engine #2 with the same issues and I assume the same mechanic, perhaps there is a theme here and he is doing something incorrectly... Not sure what it could be, but there is something going on here and I really do not think it is your radiator or water pump.