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you may have pulled someting loose behind the dash. Put a ground on the temp sensor and see if the gauge moves, if not you got to pull the center dash and see whats what
If your temp sensor is like mine ('71 sb), it is on the right side of the engine block. It has a black wire running from it. As mentioned above, the work you did either damaged that wire (in the engine bay area) or you broke/dislodged wiring when you were under the dash installing the tach cable. There is no relationship between the tach and the temp sensor...only that you were working in the same general area where they are routed.
The temp sensor is in the head on the side, kind of between the exhaust ports. It could be in either head, depending on what work has been done in the past. It has one wire on it. Take that wire off and run a jumper to ground there to see if your gauge works. You probably just pulled that wire off of the sensor. After a number of years the plug that goes on the sensor gets kind of loose.