My 428 is running at around 210 and have not had any problems. It does get hotter than that when I slow to "parade" speed. I changed the water pump and put on a 16# cap and a 180 high performance t-stat. It runs a little cooler but not much. If it gets to hot it is very difficult to restart.
The most important thing I did was to move the temp sending unit from the head to the intake manifold. Before that I was getting really high temp readings and the water did not measure that hot on my termometer.
Trans work better with hot fluid so don't cool it off to much with a trans cooler.