Water Pressure Gauge 1999 Opti 225hp

[Primal 98]

Hey guys,
I'm a little confused over the correct water pressure gauge to fit to a 1999 225hp Optimax with a Verado style LWP gearbox. From memory the 2.4/2.5s run at water pressures that require the 30psi gauge and the 3.0L engines run at lower pressures (7psi rings a bell). Is this still the case, and if so, I would assume the 15PSI gauge would be the one that I need.
Additionally, is it wise to plumb it into the Compressor cooling line as an indicator that it may have plenty of water pressure, but none getting to the air pump? The pics below are the series of gauge I am talking about.

[matt83]

I've also been wondering where to install the sending unit, if you figure it out please let me know. I have the same engines.