On these thermistors that are 120 ohms cold what should they be hot. And what would be an acceptable voltage drop in b+ after warmup, say a few minutes.

That all depends on how much current is flowing through it. Assuming you mean a CL-90 thermistor, the datasheet says about...

8 ohms at 0.5A
3 ohms at 1A
2 ohms at 1.5A
1 ohms at 2A

I have a new thermistor that was for an RCA and it's 120 ohms cold but hot it's 5 ohms so with that I'm also 5 volts less on the b+ with that 5 ohms present while running.