Ahhh, another planehead! LOL Rebuilt a few P&W 980's and a Lycoming back in the day, but I digress.
You say that with the old radiator, it ran at 205 so you upgraded to a 3 row and it ran fine. While apart, you decided to change thermostats and installed a Mr. Gasket. Now you have a problem with running too cool. You again install another Mr. Gasket thermostat and nothing changes, still too cool at times. My thinking would be to install a totally different brand of thermostat and try again.
Over the years, i've run the cheapo Auto Zone and checker and they worked fine year after year. I"ve run the high dollar ones too and they ran fine year after year. My thinking, at this point, would be a bad batch of Mr. Gasket thermostats.
Even if you have a 40 row radiator, if the thermostat is working correctly, it will hold at the rated temp. Also, 15 minutes for warmup is not out of the ordinary. My 350 takes about that long to get up to temp. My other truck is up to temp in about 1 1/2 minutes!
Start with the easiest one first AND were things seem to have changed.....thermostat.