Thermostat for Rangers is rated as 190-195degF, not 160 or 180degF
Simplest way to refill without trapping air is to remove 1 heater hose at the firewall, the one from upper intake is best but either will do
Fill until coolant/water starts to come out hose and/or Heater Core
Put hose back on
All air is now out, or enough so any remaining will end up at the top of the radiator after thermostat opens the next time.
Center of Ford Temp gauge is about 200degF, so after warm up needle should be just below 1/2, above 1/3 for sure
Make sure thermostat is working without leaking:
Start cold engine and feel upper rad hose will be stone cold of course
Turn heat control in cab to HOT, fan can stay off
Wait 3 minutes or so and feel heater hose, should be warming up
Feel upper rad hose........should still be COLD, if it is warming up like the heater hose then thermostat is leaking
Upper rad hose should stay cool until engine temp gets to 190degF, then it will start to open and hose and rad will start to warm up
Not that it matters now but most thermostats come with a "jiggle valve", yes that is its official name, lol
It a small hole in the thermostat plate with a loose metal pin in it
This hole should be mounted at 12:00 position, highest spot, it is there to let Air out as you refill the engine cooling system or to let any trapped air out after engine heats up and cools down a few times.
It is called a jiggle valve because coolant flow causes pin to "jiggle" in the hole keeping it open all the time.
Next FYI, Reverse the heater hoses at the Firewall every few years, this keeps the core cleaner by back flushing any debris out the way it came in