older 6v car, use 12v battery?

Possibly a Power inverter to power the Schumacher battery charger.

I'm thinking of getting one of these DC-DC converters

http://www.powerstream.com/dc-1206-11.htm

By my theory I can use the 12v alternator (already own) and 12v battery (easy to get) and use the converter to charge and power the 6v battery and components. I'll still have to bump the prices against a full 12v.

There is a difference between a "power supply" and a "battery charger".  If you hooked your 6vdc battery up to a power supply chances are you will eventually over charge the battery and cause it to fail, or worse explode.  To do what you want you need a 6vdc battery charger that operates off of 12vdc.

If you wanted to completely eliminate the 6vdc battery it would be a bit easier, as you could then use a 6vdc power supply without worrying about damaging the battery.  The only big downside to that is the fact that the power supply won't be 100% efficient so you won't be able to keep it on while the vehicle is off.  You would need the power supply to come on when the ignition came on (so that you could power the starter) and be able to provide enough current to turn the starter.

It may cost you a bit more up front, but I would think a full 12vdc upgrade would be beneficial in your situation.

Cool I see what your saying. I've been reading up and I think I'm going to go ahead a put the 12v battery and alternator in. Then just force feed the starter 12v till I get the solenoid replaced. I've pushed it a bit with 12v@6A with no ill effects so far and I've read on other sites of folks doing this with their cars. I'll look into 12v powered chargers or a use a relay on a power supply.