Pressure sensor for a 2.5 litre

[HumberJewel]

I want to add a water pressure gauge to a 2.5 litre 200 hp Merc.
It has been suggested that rubber hose between the two thermostats is the place to take the measurement from.
I am not convinced. It appears to me that when the engine is cold and the thermostats are closed, that there is no flow in this hose and therefore no pressure on this hose. I think the water pressure should be taken ahead of the thermostats
More confusing is that this hose also is the water source for the tell tale (pee stream). Is there a pee stream when the engine is cold?

There is also a hose that goes from the bottom right side of the powerhead to the top of the powerhead. What is the purpose of this hose. It is it good spot to tap into for a water pressure measurement? The red X's represent the thermostats.
Please enlighten me.

 

[Texasmark]

Agree!

 

[boscoe]

Where does Mercury show the connection for a water pressure gauge?

Looks to me like it would be in the vicinity of the exhaust manifold cover. Where water is first coming into the block.

 

[HumberJewel]

The thermal air valve port looks like a good spot. The valve is not in use.
What do you think?

 

[HumberJewel]

Nope. It is a blind hole (thermal well?). When I pull the heads (waiting for gaskets) I will see if I can drill into the water jacket through the side of the thermal well. In hindsight, the blind hole is obvious on the other head.