You can reverse the polarity of a battery if the charger is hooked up wrong. You didn't say if the battery had been charged - I'm just guessing it was.
I'm gonna enjoy the steak!

LOL - Couldn't have put it better myself! Test the polarity of the battery. Make sure it'* reading +12 volts not -12 volts when the voltmeter'* red lead is on the positive terminal. If it is reversed then throw out the battery. I wouldn't try to 'rereverse' the charge.

Just stumbled onto this topic,

Seen this happen with one of my Sterlings I used to drive

I agree reversed battery polarity is a good possibility..

Paul...pass salt will ya?

remember, the starter might have a direct link to the battery + aswell, if it doesnt have a fusable link in the Starter cable your short could be down there

i dont see how a reversed abttery would arc out real bad, but maybe im not thinking of a reason that it would

Argh, even as inept as I am, the first thought that came into my head was "sounds like they hooked up the battery backwards", but no other possibilities came to mind. Oh well, I'm not going to WCBF anyway.

Is it posssible that the batteries have internally shorted?

I am assumin that you have 2 12volt batteries in parrell, using the body for 2 batteries negative connection. Make sure that they are hooked up properly.

Is the alternator installed correctly

I would hook up both batteries and try and start the thing with out the alt and see what it does.

I believe there is a diode inside the alternator that only allows current to flow in one direction. Reverse the battery polarity & you have a direct short to ground. Sparks real good!

Unless something that old is externally regulated, which would mean that even if he swapped alternators, the reg is still bad and needs to be changed!

GOOD CALL!