i was going to get a new one and took out the old one. it doesn't have a temp on it. it's a stock '75 460. anybody have a geuss what it would be? it is not in the truck yet.

 

There were generally 2 choices, one was a 195 degree thermostat which brought the temp up higher and helped the engine run more efficiently plus provided better heat if you lived where it got pretty cold during the winter.
The other choice was a 180 degree thermostat which opened sooner , still provided decent heat and met the requirements for operating temperature.

My suggestion is someone evidently installed a cheap thermostat and it made no difference to them.
Good quality thermostats always have the opening degree numbers stamped on them usually on the circular bottom center of the capsule..

 

thanks hanky. i live in a desert so i will go with the 180.