There are not a lot of options here.
Either you plump a T where the stock sensor is, which won't give you a very good reading or, I don't know if it is accesible, use the pass. side head water temp port. Just like on the drivers side.
Is this a mechanical guage or an electric guage? If it's electric the sender is NPT and you can just drill and tap the water neck, if it's mechanical you can still use the water neck except you will need some different fittings. That's where mine was and never had a problem.