Yea my water pump housing has that plug, it's too small for your standard mech temp probe. Which I believe is 3/8. Was thinking of tapping it a thread size bigger or possibly drill and tap opposite side of stock gauge for temp, I have gauge setup there, and I need to drill for electric fans relay.
Here's a write up someone did.
http://www.hotwireauto.com/?page=hemi_eng_temp