Before I installed a heater, I installed the sender for an aftermarket mechanical temp gauge in the heater fitting on the top of the manifold. This worked pretty well.

When I got around to installing the heater, I put a tee fitting in the rubber bypass line under the thermostat... I used a copper tee, and brazed in a threaded adapter for the sending unit. It would just fit in the middle of the rubber hose, with extra clamps on each end. This was pretty simple and cheap, but the temperature readings seemed a little low. Then again, it may just be the cheap gauge.

I eventually drilled and tapped a hole in the manifold near the heater fitting. It may not look stock, but it is about as direct a temperature reading as I can get.

Does your intake manifold have a tapped hole near the thermostat?

I would like to install an aftermarket temp gauge in addition to the factory gauge, where would be a good (easy) place for the sender unit?

I thought of either in one of the block drain plugs, or the unused plug on the water pump.

South Australia