If you're not running a tach, then you only need one wire. That wire should be switched by your key. Yes, you can temp it to the battery, but you'll have to pull it off the turn the engine off. Not a real good idea though, if you happen to have a problem at the same time (I'm imagining an engine fire and you can't get in to pull it ). Personnally, I'd temp it though a toggle switch, just in case.
On my HEI, I think the right terminal is the power (has a + on the housing above the terminal).