Not sure what you mean.
There are volt meters for bikes, ones you stick on something and ones that clamp on handlebars.
You just connect to switched voltage. There are LCD ones that use so little power you could connect direct to the battery (with a fuse!) and leave on all the time without any worry about discharging - they use power like LCD watches that can run for years on a button cell.
Connecting to any black wire and ground will have it switched. You won't read the true battery voltage this way - always reads a bit lower - because of the voltage drop through the wiring harness but it will give you an idea of what's happening.
Adding a small relay at the battery with the coil on a black wire, then switching true battery voltage to your meter will tell you the real battery voltage.
Probably you're having charging problems, to want a voltmeter? That's not unusual for a 350 or 400, the alternator is not oversized and any voltage drop from dirty harness connectors or switches can cause poor charging. Using a higher wattage headlight and/or some electronic ignitions can overtax the alternator and leave you with a chronically weak charged battery.
