I seriously doubt you are going to find a sender that small that will work with the factory gauge.
If your engine has a 3/8" NPT threaded hole in either head, you can get one of these that will work with the factory gauge.
http://www.67-72chevytrucks.com/vboa...d.php?t=726700
If the heads do not have any threaded holes for a temp sender, What I have done before is install a tee fitting where the heater hose outlet is on the intake manifold. Then install the factory original style temp sender in the top. This is not ideal but at least you have some way to monitor the engine temp.