Original question answered.. negative ground.

But, it often just doesn't matter. Cases in point...

My old Jaguar was originally positive ground. As I was told, I just hooked it up backwards, the generator polarized itself fine, all electronics were fine, and it allowed me to put a modern stereo in there.

Being the sometimes sloppy shadetree mechanic that I am, I once put the battery in my 12v '62 544 backwards. Again, the generator polarized itself for positive ground, and everything worked fine. Except the modern stereo, which got cooked.

1950 dodge truck. Didn't care (was originally pos ground), but the generator never worked. Don't know if it was because I had the battery hooked up backwards or not.. but I'm guessing not. Guessing it was that mice had eaten all the wires. Modern stereo? yea... right.

I'm not saying try this at home... But changing from positive ground to negative is doable in vehicles with generators, often doing no more than hooking the battery up backwards, and it allows for the use of modern electronics.
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-Matt I ♥ my ♂