Are there any good places on the 2.3 block to install a second temperature sender? I'd like to keep the factory idiot light while also having an aftermarket gauge.

It may not be the same on a normal 2.3, but in my observations, coolant temperature of a 2.3T varies a lot between the different access points. It's going to read much lower at the back of the block than it will at the intake or thermostat housing because none the coolant has gone through the head yet. I have a gauge sender in the block and it reads about 30-40° lower than the EFI's sensor at the intake. That's the difference between 195 and 230. 

I would be inclined to put a sending unit as close to the engine's coolant outlet (t-stat housing) as practical.

On a side note, rust probably collects at the back of the jackets because the coolant velocity is lower and it changes direction there. There might even be little reverse-moving eddies there that drop suspended particles.

I wanted to monitor the water temp at the head so I installed a brass tee in the water outlet from the intake manifold to the heater and installed the temp sensor there so it would montor water temp right out of the head...

See Attached photos

I used a short brass nipple into the manifold, a  brass tee connector to the nipple, a 2 inch nipple into the center of the tee,  connected the hose to the heater to this nipple and installed the temp sensor into the other "straight thru"  opening in the tee.

I also replaced the heater hose with one more straight forward and added 90 degree elbows as shown in the photos.

the fittings are all standard plumbing fittings and the copper tubes are plumbing as well..