TH400 speedo gear

Not near enough info to answer that one.
Might just need the driven gear but depends on what drive gear is in there now.
Have some info:
http://chevelleengineer.home.comcast.net/~chevelleengineer/speedo.htm
http://www.chevelles.com/shop/speedo.html

Still not enough info.
Check the links I posted for what you need to input to find the correct gear/s.

Sounds like you need to change both gears to get the ratio you need.
Alternative is to have a speedometer shop build you a ratio adapter.
This way you don't need to pull the tail housing to change the drive gear.