Remove one of your block drains and install a Tee. Put plug on one side and a compression fitting (1/8 NPT to 1/16 compression) on the outer to use air brake line up to the gauge. Some have installed it in the thermostat housing and I still need to remember to move mine from the block up there for pressure comparison for tractionless

Note: the above is for open cooling

If the motor is closed cooling, then need need a serial number to see what you have

On 700's (Fountain Lightning), there is a t-fitting on the oil cooler, on the back of the engine. The t feeds smartcraft and mechanical gauge. It's just a brass block. I might have a pic somewhere...

Here's a pic.

Nothing wrong with an water flow meter for an install. Some Marine oems are installing them instead of water pressure system / monitoring or both. Water pressure and or any pressure reading is a form a resistance. Sometimes it can fool one when monitoring.

Example a low water pressure reading can become a high water pressure reading when a line or orfice after the pressure reading becomes restricted.

Actually I like to have both readings -- water flow and water pressure

Here is the Tee fitting which can be used on the oil cooler
https://www.boats.net/product/mercur...gaAo3eEALw_wcB

Might also find it at a hardware store