and that valve connects to the motor via a short length of stiff wire (as the linkage). That wire is removed as part of the process when removing the heater core. I recall the valve NOT being easy to operate when connected to the motor but with the linkage undone -- was no issue.
Main issue will be getting to it -- just to the right of the gas pedal 3-4 inches and up maybe 9 inches. That's 9 inches above the tranny hump. Get a flashlight in there - find the coolant line pipes (in/out) and you should be able to locate the valve. Yes - not as easy as on some other trucks/cars...