Where is the best place to take the water preasure on the engine? Head , block, intake water runner...?

Don't think it really matters..somewhere after the sea water pump and before the exit at the block. Usually a fitting somewhere in the intake manifold is the easist way to rig a tube to the dash gauge.

Top of intake so it doesn't get plugged.

+2 Intake, Blocks get plugged.

Some oil coolers have a 1/8"NPT fitting on them that might not be a bad place if you don't want to drill the intake.