I can't speak from direct experience with a 70, but my engines (90, 175, 200) have all had around 5 psi at idle and somewhere around 20 psi at speed. Sounds like yours is doing good.
You might want to consider a cylinder head temperature gage, easy as the pressure gage to hook up and a good companion for the pressure gage; maybe 150 deg. at idle, 170 at mid range, and drop down to the low pin at high rpm.
OK. I never hooked up any of the gauges in my boat, they were all factory. The boat is a 94 LOWE 1730 fish and ski. It came stock with a fuel, rpm, speedometer, trim, volt, and possibly one more?
When looking at water pressure guage you often get a wrong reading realy, as most guages are from a T-piece on the tell tale pipe.
that being on the T-piece means you are at maximum only getting half the reading of the real pressure.
if the pipe from the guage goes directly to the bloke then that is different.
Just 1 pound of water pressure is enough as long as your getting the volume needed for the engine.
Myself i prefer to have a temp guage to tell me whats going on, you dont see water pressure guages in cars or trucks just temp.
The pressure guage can give fals sense of security as recently a friends engine had plenty of water comming out the exhaust
yet one bank of cylinders was not getting cooled due to bad thermostats stuck closed. this would show good pressure yet be very
innacurate as to whats going on.
to be bsure then fit both a temp guage and pressure guage like mentioned above.
