Well firstly it's a dangerous game to start number chasing, or should i say costly.

Imo if it IS 400 bhp you're after i would buy a T36 turbo, or a 3071 and run them at less boost to get your goal rather than ringing the neck off a T34.

As for Spec, Radtec, Pro Alloy cooler, 55s injectors, clutch, Small Turbo Escos gasket will be fine, brakes are a must imo, group A coil, Level 6 or 8 ECU, live map ( again imo) MSD chip is good also. Breather system, cam?? Etc etc

You could also do what Andy has said above and go with a T34.48 /55 /63 , you can get close or beyond 400 just but imo as said it'll be ringing the turbo's neck. A nice genuine 370-380 bhp Cossie with similar torque is a great little road car

It all starts to add up.