i have not long bought a 300zx twin turbo jap import with auto box and was wondering what the most reliable way would be to get around 400bhp and any recomendations for upgradeing the brakes

I think one of the best ways to get 400bhp out of your Z is to subscribe to the forum. As for the brakes searc porterfield pads.

at the crank is not too hard on a healthy engine:

 

induction, exhaust (decat will help), chip, boost increase to 1 bar,underdrive pulley should see you around 400 ish.

 

if you want that figure at the wheels then bigger tubs/injectors etc is needed.

 

whatever route you will deffo need an uprated auto box oil cooler & even then the box may not last long! plan for a fast road rebuild!! (or manual conversion ;) )