They sell a resistor for the blower motor and it is encased in metal. If you use it, you should attach the case to some sheet metal so it can get rid of the heat. The value will be around 1 ohm and it should be rated at 40 watts or more.

1951 GMC 1 Ton Flatbed -- It is finally on the road and what a great time I have driving it!
1951 1 Ton Completed

My Chevy Master 4 Door is on the Road!