I'd like to avoid having to pull any freeze plugs etc.  Is there an alternate spot to put a sender?  How about somewhere in the radiator return hose or something novel?

When testing Y’s on the dyno, I use the threaded heater hose hole in the intake behind the thermostat for the engine temperature reading.  That’s the ‘real’ temperature.  What Mark suggests is doable if also running a heater but using a Tee at that location is also an option.  Don’t be surprised at a 10°-15°F higher reading at the front location versus what is seen at the rear sending unit location.