I'd suggest using a speed control resistor from a later model vehicle in series with the 6 volt fan motor, one that has several connectors and is mounted in the ductwork with the blower air flowing over it to keep it cool. The wire-wound resistor in that setup can be tapped into at 3 or 4 different locations until you get the blower speed you want.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
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