Based on your model number, that should be a 1977 70hp Johnson motor. Johnson didn't make a 70hp in 1970. That part is your power pack. It takes the charge from the alternator and takes the timing from the timer base and sends a electrical charge to the coil at the correct time for ignition of each cylinder.
Basic rundown of positions
1) Output to cylinder 1 coil
2) Output to cylinder 2 coil
3) Output to cylinder 3 coil
4) Connection to Stator
5) Connection to Stator
6) Kill wire from ignition (grounding this wire stops output to coils)
7)
8 ) Input from Timer base Position #1 (triggers Output to coil #1)
9) Input from Timer base Position #2 (triggers Output to coil #2)
10) Input from Timer base Position #3 (triggers Output to coil #3)
11) Connection from Timer base
12) Ground
Don't wire it to #6, you'd never get your engine to start You wouldn't want to use any of the connections on your power pack for the voltmeter, none would give you an accurate battery voltage.
To use a volt meter the way you describe, you should attach the positive to the switched power from your ignition switch. This is the purple wire coming off of the big red plug. This one will have no power when your ignition is switched off.
You'll have to decide if it's easier to access this directly at the switch, or run it back from the motor.
The wires you are looking for are already run to the control box. Gray lead is your tach. lead.
Water pressure does have to be run, via a hose, directly from a port on the engine. There is no electrical sender on the engine for that.