A later-type speed control resistor could be adapted, one that sits in the airflow from the fan to keep the connectors from melting. It would require some creative wiring and possibly the addition of a high/low speed toggle switch hidden somewhere under the dash. That would involve cutting a small rectangular hole in the blower duct work to mount the resistor.
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
Love your enemies and drive 'em nuts!