IIRC its a 3/8 NPT but I'm not 100% on that. Its on the front left of the lower intake manifold. To keep the stock gauge working, you can get a filler neck with a stock water temp sensor bung and lengthen the OEM wire to run it there.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.