Is it worth changing to a 2 speed diff and what kind of increase road speed will it provide? Is there another method of shifting diff range than the vac diaphragm, maybe an aftermarket or more modern servo motor that would move the shift lever?

 

It really depends on what rear axle you have right now. The highest ratio I remember hearing of was 5.14 available in the single speed axle on the F-4s. The high ratio on the F-5 & F-6 two speeds were not that high. Find out what you have and plan from there.

The two speeds truck axles in the 30s were cable or rod shifted. You could always make up linkage and go that route.

 

Mine has 5.83 and 8.11, I also have 38" tall wheels. Those gears with smaller tires will give you a faster off the line speed, but you'll top out at like 35mph.

 

I believe I also have the 5.83. Don't know what the 2-speed is yet