What do you change to put a th400 in place of a 700r4?

Probaly the only thing that will need to be changed is the length of your drive shaft. Usually a 700r4 is longer than a turbo400.

Also, the slip yoke on the driveshaft for the output shaft on the TH400 is larger in diameter than the 700R4.

You will need to install an electric kick down switch in the throttle linkage for passing gear on that TH400.