ATF usually starts degrading in the 260-270 degree F range. In extreme cases, it can puke the fluid out the overflow vent or up the filler tube. The temp might be fine during acceleration and during cruise, but until the converter goes into an engine braking condition the fluid temp might not climb too much.
The engine is designed to run with a 180 degree F spec thermostat. A friend made an inquiry to Edelbrock years ago and they confirmed that while minor power is to be gained by going to a 160 degree F thermostat from a 180 degree F thermostat, engine wear increases when the temp is below 160 degree F. Hence, it's better to use the 180 thermostat.
There might be comments about taking a thermostat out to allegely cure overheating, but in reality, the thermostat is there to not only regulate the coolant temperature but to also restrict the flow of water so it doesn't go through the motor too fast and not pickup enough heat in the process, not to mention the somewhat regulated amount of turbulance of the coolant as it flows between the cylinder bores in the block. In some racing applications, the thermostat is replaced with a washer with a certain size hole in the middle, but they don't have to worry about coolant that much as the races are pretty short.
With age, deposits will accumulate internally in the block. The only way to get them out is to pull the core plugs (aka freeze plugs) and flush the whole thing out--a messy job, but beneficial in many cases on older vehicles. Similarly, a radiator core might look reasonably clean on the top, but could be pretty much plugged up at the bottom of the tubes.
Propane can result in different power output due to the nature of what it is. Some municipal fleets use it and similar gases, but there have been comments about decreased performance. Seems like there was a drag racer in the early '70s that built race engines around propane and they worked, but they were highly different from gasoline fueled engines.
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