I wouldn't worry about the switch at high speed, as at high speed it's providing a short (zero ohms resistance).  I'd be more concerned about the low speed on the switch (as the resistor is now handling twice the voltange, and hence twice the power) and also the fan motor.   

Sending the full 12 VDC (or more likely 13.5 Volts) to a motor designed for 6 VDC sounds like a formula for increased heat in the motor.

427carl wrote:

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Or just run it, they will run at a higher speed that they are designed but will usally last becuase the 6 bolt units have heavier windings, you can get universal heater switches at an auto parts store and just use that to turn down the speed. but they arent built to a very high quality and those dont last, even on a regular 12 volt system, blowe motors pull so much current they tend to burn out switches, that why they usually have blower relays on em nowadays. you could figure out how to wire in a gm blower relay and switch from a 70's era and that would probably work fine.

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 Was hoping to run the original switch to keep dash "sorta' stock even though the cluster is custom. As long as it will work for when safety is done,then it can "burn out"

The one above cost $27.99, this one from Speedway Motors is cheaper but shipping will be more:

Electric 12 Volt to 6 Volt Reducer for Motors

 

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This voltage reducer allows the use of an original 6 volt heater blower and switch in a vehicle converted to 12 volts. Safer and much more reliable than the less expensive ceramic type voltage reducers. Compact 1" x 1" x 3" size occupies little space. Reducer works with up to 10 amps of continuous current draw.

Note: This reducer generates a fair amount of heat when in use. It should be mounted to a non-flammable object, preferrably near cool moving air while the vehicle is in operation.

910-64103 Instructions (PDF)

Implication from the comment about less expensive ones is that you should be able to find something even cheaper somewhere, eh?

Dave