Guys,
I am bulding a 69 ford f100 daily driver. I have just finished a 351 windor that has some nice mods. A c-4 is not going to hold it and pull my trailer. I have a local tranny shop that is going to build a c-6 for me. However I need to know what speed o meter gear I need to put in this c-6. Thanks

 

Wrong section... This is the 87 to 96/7 section

But the speedo cable fits both the c4 and c6. There are a few different inner rings to drive the cable gear, but those only need to be swapped when you're going radical rear end gears.
The cable gear you need depends on your axle ratio and tire size. I'm sure there is an online calculator or chart that can help you out. But for example, I run a 23 tooth gear with 410s and a 26.6" tall tire and its pretty close.