edgardo - From what I've been able to find on original 1960 235 electrical temperature sensors, if your head is an original 1960 piece (check the casting date codes and number on the head on the top drivers side area near the carb and post them here to help us confirm), the port for the temperature sensor should be a 1/2" NPT (tapered thread) opening.
According to various Chevy Master Parts Books, your 1960 Chevy C10 235 should use coolant temperature switch assembly GM# 1315521. If your port is in fact a larger opening requiring a coupling, the coupling you have should work with this original GM sending unit to provide the correct signal for your original dash gauge.
Lots of various vendors offering original NOS pieces, just do a web search using that number and you'll find what you are looking for.
