I'm not sure how you did that, all the mechanical gauges I've ever seen were far bigger than the 1/8 pipe thread of the stock sender.
You MIGHT be able to drill tap the manifold out to 3/8 pipe and get an adapter (if not suppled) for the temp gauge. These are a special fitting.
Otherwise, one way, if not ugly, is to remove the heater hose fitting, put a pipe nipple / tee in there and screw the temp gauge in one leg and the heater hose fitting in the other.
See if your rad has a fitting, some do. Trouble with this, is if the rad leaks down, the temp gauge may not warn you it's getting hot.
There also was a VERY ugly deal that you could tap the upper hose with. I've seen two versions, one was a metal sleeve, you cut and put in the hose, and the sleeve had a fitting in the side.
Old school was a little pair of curved brackets/ washers and you simply cut a hole!! in the hose, clamp the fitting in there with the two washers, one of which is threaded for the fitting.
I've got some Edelbrock manifolds, and it ticks me off that "Vick" could not be bothered to provide us with a nice 1/2" pipe fitting fer crimeny sakes.