There are a couple of people that make thermostat spacers, with bored/ tapped fittings out the side. You put that under the stat, and the gauge sender into one of the fittings. Mancini, I think, is one.
There used to be fittings that allowed attachments into a radiator hose, but they can fail. Otherwise, you have to be selective with fittings, and fiddle with a tee in the heater hose connection--the one that is below the stat, flow wise
Or change to an electric gauge, which, generally, use smaller senders
