I would be more concerned with using a 6 volt coil with a 12 volt system than what the dwell is. The proper way for a 12 volt system to work is with a ballast resistor and a 12 volt coil designed for that system. You are likely not getting the correct voltage for the engine to run properly.
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