I've always thought the three brush setup was a Rube Goldberg design and really had difficulty keeping the battery up in the winter time. I had a 39 Master 85.
If you're not totally wedded to originality you could go to the 1940 setup that was used through 48, they look almost the same but you would have a regulator on the firewall unless you hid it somewhere else. The later design required less field current and by '49 output was up to 45 Amps at 2400RPM as compared to 20A at 3400RPM.
I used to pull a trailer with my 51 and had set the current limit up to keep up with the trailer lights. This was more than the Delco generator could handle and the solder melted out of the commutator. Close proximity to the hot exhaust manifold probably didn't help either on long pulls over the mountain passes. So I put on a 45A Autolite generator off some old Chrysler with the same Delco regulator. I only removed the Autolite generator when I converted to 12V and an alternator about 25 years later. The battery always stayed charged even with a larger pulley as the Chrysler generator output was rated at 1000 RPM instead of 2400.
