43 Marmon Herrington 4x4 top speed

[42Ford M-H]

Any info on a safe top speed for the 4x4 drive train? The war tag says a top speed of 40. I'm not sure where they get that number from. Gear ratio? I'm shooting for 55 mph once I change gear ratios etc... or maybe even stick in an overdrive. I'm assuming if i take it easy that 55 won't damage the 4x4 components but I don't know that for sure.

Thanks in advance for any info.