Overdrive for 3-speed

[rickf]

I've seen a couple mentions of Chevy 3-speeds with an overdrive. I'd like to know more about them.

My truck is a 64 C10, 3 on the tree.

What years/models of vehicle would come with an overdrive that is compatible with my truck's 3-speed? (If any.)

Does the overdrive directly replace my stock tailshaft?
I mean, will it bolt right up, is the same length, same bushing diameter, same splines, and my same drive line is going to fit?

Could the tailshaft/overdrive exchange be done with the transmission still in the truck?

What was the normal in cab control for the overdrive?

My C10 does indeed haul its share of 1/2 ton loads, will the overdrive hold up?