My new project

Finally got a day off today and bought a few gauges as some of mine didnt work(oil pressure and fuel guage and amp guage).
So I bought a 3 set with oil psi, temp, and volts) . I plan to use a brass t fitting with a couple nipples to splice into my heater hose for the temp guage because when I used a pipe wrench on the hose fitting that's threaded into the head it wouldn't budge at all and I'm afraid of damaging it.
My question is this, how many of you all have done this(I've seen a few good photos of what I tried to do) and about what difference in temp should I expect if I had been able to thread the sensor into the head instead of in the hose 3-4 inches away?
10 degrees? 15 degrees? Also it's a non ac car so I believe water should be flowing constantly in the heater lines correct?