you'll want to swap the diffs back if your after more acceleration.
auto- 3.9-1
Manual - 3.7-1
or, if you can find them, pintara pinion and crown wheel for 4.1-1 ratio
and with the speedo issue, the 31s and 33s both have electronic speedos. you need a Speedo Interceptor (jaycar sell them) and the just adjust it using a navman or something.