How brave (foolish?) are you?
I bought a supposedly 6v wiper for my '54 from Truck Shop. Besides running too slowly, it also got in the way with the speaker on my stock radio (there are no aftermarket electric wipers that work in a '54 truck with the standard radio - the GM 6v 54 wiper works fine with the radio).
Truck Shop told me that they sold a 12v wiper as the 6v wiper (and they said that most people found that it worked OK).
If you are brave/foolish, try the "6V" wiper with 12v (remove the reducer). Run it briefly and see if it works.
(by the way, I take no responsibility if you try to run a 6v device using 12v)