Output Transistor Heatsink Temperature

Am wanting to install a snap switch to audio output heatsink on 600 watt amplifier to turn on a cooling fan when heatsink gets too hot. Not sure what would be an appropriate temperature for the snap switch sense. Thinking about one that would close(turn on fan) at 120 F and open (fan off) at 105 F. Should I go higher before turning on the fan, or what is a good temp for the heatsink to not exceed? The outputs are DAT 1521 and DAT 1018, which I cannot find any data sheets on.
Thanks

50 to 75deg C

ok thanks, so will go with on that turn on at about 140F (60 C) and off at about 120F.