No problem switching to negative earth for your new alternator. There was nothing in the stock Packard which was polarity sensitive. Many cars were accidentally changed by a mechanic and sometimes the change was not even noticed -- other than occasionally the generator would not charge with the switched arrangement and had to be corrected.
The only polarity issue now would be if your car had a repair on the radio in the last few years. If a new solid state vibrator was installed, that item will not work and would have to change.
In addition to what you have changed, the starter solenoid may also have to change and you probably should have a ballast resistor for the coil. Don't forget about heater motors or any other 6v accessories.
There are some voltage converter devices such as this one
http://www.ebay.com/itm/Voltage-Reducer-Regulator-12-VOLTS-to-6-VOLTS-20-AMPS-/140750092717?pt=LH_DefaultDomain_0&hash=item20c55bf9ad&vxp=mtr to keep 6v accessories running with a conversion. They might be worth a look.
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