I didn't see this mentioned, so I'll ask.

With the engine having been run for at least 1/2 hr (brought up to normal operating temp)
can you touch & HOLD the 2 hoses that come from the heater core without getting your hand burnt?

 

pull the heater box pull the fan look inside at the door it may be stuck and the blend door axle is just moving without moving the door

 

i just went through this exact problem with my 78 Bronco.engine ran a constant 195 which is the thermostat but no heat.i checked the core flow which seemed fine.so i changed the pump no diff,rad ,nothing, i then bought a new core did a flow test on both .turned out the core in the truck was flowing but not enough to heat up to give heat.the blower was cooling it down to fast.put the new core in and now i am driving around in just a shirt swetting.for the price of a new core i would change it u may have had the same prob as i did