I would pull my volt/mph/hours gauges and put in temp/water-pressure/oil-pressure if I only had space for three.

perhaps you can find a dual input split gauge (one with two indicators on one face, or find a digital gauge that can be configured to have multiple displays like this and use some pressure senders instead of mechanical

https://www.ebay.com/i/293747613516?...4aAkVvEALw_wcB

My dash mount speedo always gave me problems, anything over 60 mph and the pitot would flip up. Found I was always looking at the chart plotter for GPS speed anyway. So I rerouted the speedo hose to the engine water jacket and now the speedo tells me block water pressure. But if I was going to add a gauge to the dash, I think exhaust manifold temp would be more useful.

I'll find a way for a second gauge and ditch the engine bay gauges.

As mentioned above, if raw water cooled - take your water pressure from intake manifold. It will be less likely to clog.

I'm going to just add a second sending unit for my oil pressure gauge with a SPDT switch. Left is water pressure, right is oil, easy enough.

Can you take a photo or something to show this location?

Tom,

Yours should be similar to the HP 500 (this basic set-up has been used from the HP 420 forward). Remove the plug indicated by #3 in the drawing and adapt your line to that.

Perfect, I now know where to look thanks Tom Z

Is the preferred method of attaching the water pressure gauge through the 47440 thermostat housing or directly to the Hardin pump?
So, I think I found the correct port on my thermostat...agreed? What size adapter should I use?

I have some Gaffrig water pressure gauges that came with these connection adapters...which are too small to go directly into the thermostat.

I did notice that my Hardin water pumps seem to have a unused port already (that looks like it fits one of the connectors/adapters I have):

That plug looks like it goes to the fluid reservoir for the fuel pump.

Use adapter bushings and take the pressure reading from the thermostat housing. That will give you the best indicator of pressure at the engine (will reveal blockages between the pump and block).