You didn't say which transmission. You'll need your rear end gear ratio and your tire size to figure your speedo gear set out.
There is a drive gear in the transmission and a driven gear. The driven gear slides into the hole in the side of the transmission along with the "bullet" cable drive. The drive and driven gears must be compatible.
Hopefully, and most probably if the transmission is from a C10, you will only need the driven gear, because a change of the drive gear necessitates going into the transmission.
There is a ton of information and calculators on the web on determining the correct gear set. Here's a good thread to get you started:
http://nastyz28.com/threads/muncie-s...e-gear.224283/