when I bought my most recent sammy it had an aftermarket temp gauge installed on it. The sensor for it was installed in a different spot than the stock temp sensor so both could be used. The thing is that for some reason the aftermarket one never worked even though it appeared to be installed correctly. I don't know what kind of threads it used though but it was only a little bit bigger than the stock one. Sorry if this isn't much help. It doesn't look hard to do though, I think mine uses an adapter of some sort to mate from the smaller size hole to the larder size senor for the gauge, so on that note I'm not sure that its totally necessary to even tap the manifold. If I had a red star I would just post a picture.
hopefully you can take something from this