I'm thinking it's a little low too, but then again, it's at idle. We haven't established whether there are underdrive pullies on it, which could be causing a lower alternator output. How is the voltage over the normal operating range of the engine? Does the voltage rise to ~14V when the engine's running around 1500rpm?
I think it would be great to find out how well that cable is transferring power to the rest of the electrical system, and also how well the battery's negative connection is made, i.e. is everything crimped & bolted down, or are things soldered, shiny, and
? This is one of the things that kinda scares me away from trunking my battery in the notchback: the gauge of the cables is going to have to be huge to ensure minimal voltage drop from the battery to the starter & alternator.