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I've been struggling with a broken off temp sensor on my 39 120 since I bought it years ago. Someone had put a steel pipe reducer in the block, rather than the brass one I'm used to. In the distance past, the sensor was broken off in the block. The car is running, but I really didn't want to pull the head unless I had to. The clearance is tight to get to the sensor due to the steering column. I found that by using increasing sizes of drill bits in a right angle drill, I was able to get up to about a 3/8 hole. There I was stuck, as the drill bits for the larger sizes were too long to get in the hole. I found that there is a type of drill bit called a "Silver and Demming". It has a long shank that will fit in a 1/2" drill. The cutting portion of the drill is about the same lenght as the shank. So I bought a set from my favorite "use it once and toss it" store, Harbor Freight and cut the 1/2 and 5/8 shanks down with a cut off tool, then slowly drilled out the remains of the old steel reducer and the brass sensor. I was then able to tap the block using a pipe thread tap (a good one from Sears). After cleaning up the drill cutting with several magnets as well as by adding water to the radiator to flush out the chips. Hopefully I've gotten them all but I installed a stout magnet on a wire in the radiator just in case. Hopefully this will work now. FYI, this trick won't work on a 51-54 model as the firewall is concave, preventing a clear shot at the sensor hole.

On the 51-54 the sensor is the smaller electric style and screws right into the head.