JimboTX
Nowhere is your discussion did you indicate that this "6V to 12V conversion" had a ballast resistor installed. Or during replacing parts was it mentioned. HRL is spot on if you want this truck to run correctly.
Note: Buy a ballast resistor for a late 50's truck and a 12 volt coil that has the words stamped on it: For use with external resistor.
Fred

1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine