Your voltmeter is reading your entire charging system. It gets 24V via the 10amp/28v fuse at the bottom of the fuse box (and cuts it down to 12V with a resistor on the back). You say it's going into the red past the green (high red, not low red)?
First, answer Warthog's question. The driver-side alt doesn't have to be isolated ground like the passenger-side alt, but they both need to be fairly heavy duty 100 amp alternators as far as I know.
If you did indeed use a suitable replacement, use a multi-meter to check your voltage at each battery both while engine is off and while engine is running.