I ran it off the hose that goes from the radiator into the surplus tank/ filler tank. Brass tee, two hose bibs and the sensor on the third threaded port. Seems to get plenty hot, plenty quick. I could have swapped out one on the flange for a location off the head but I didn't think it was necessary. Hot is hot. Water is always moving in the system, and not routing down through the radiator until the thermo opens. So I feel temp is recorded well enough and soon enough to avoid an overheat incident. If I really want to know the exact temp I would move one of the WVO temp sensors against the head and measure it. Those read up to 183 degrees but I would be able to see how fast it gets there.
The small line - like 1/4 inch id on top? Or the bigger one on bot 3/8 id?
Top line it be the hot side in. Lower one is cool for too long. I suppose if you really cared to get close to the source you could tap right off the radiator a few inches rather than loose a couple tenths of a degree along the way. I just had enough wire to make the location next to the surplus tank. I used one wire to the sender and grounded the brass tee with a second to the same screw on the top post of the shock mounts. I used an aluminum ring with a wire take off. Compress fit on the sensor to brass tee. No leaks, yet.
I have mine in the upper radiator hose flange right in the head basically. I can watch my gauge jump all over based on what is happening. Even just starting to move and getting air through it, I can see it drop quickly.
I'm running aftermarket gauges in my rabbit pickup.
For water temperature, I use Acmeadapters gold rear head water outlet flange. Mounts just like stock and has a port for an aftermarket water temp gauge.
For oil pressure, I just use an 1/8th pipe thread & brass "T" in my oil pressure dummy sensor port on the oil filter housing. Threads are very close to being identical. Both gauges work great.
I do all my diesel converted Suzuki's this way so I know exactly what the motor is doing regarding temp and oil pressure.