The stock 49 radio will work with negative ground. If modified and the vibrator has been changed to a modern solid state replacement or any other solid state conversions such as FM, Bluetooth etc has been done then it will be damaged if polarity is switched.
When doing the OD solenoid conversion don't forget about the relay. Instead of the expensive original looking R11 repro you can use a modern inexpensive 30 amp Bosch type cube relay. If the original relay is shot the guts can be removed and the Bosch hidden inside the can but if the original relay is still good it would be nice to save it as ebay material for someone looking for a 6v original.
You can buy a dropping resistor similar to this one
http://www.fifthaveinternetgarage.com/electrical_parts.php for the heater motor. Place it in the wire feeding the heater switch and it will work well on that application. Just be sure to mount it solidly and in free air away from any wiring or other items because it will get hot. If stock, the heater feed wire is probably off the ign switch so inserting into the wire over in that area would work well.
You could get Runtz regulators for the instruments but it would require separate ones for gas, oil and temp so that would be expensive. If you are building your own that is fine but there are 12 to 6v converters capable of handling several amps available on ebay. They would be good for instruments, bulbs and the like but unless they specifically state it is OK, most are not for use with motors or solenoids.
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