king bearings are my first choice for quality
darton sleeves
crower rods
lowering compression will mean you make less power. but on 20 psi, you should be making quite a lot of power anyway so the lower compression pistons are probably necesary. either way you need forged pistons.
stick with the stock head gasket.
stock cams are usually the best bet for turbo. although you will need cam gears to dial out some duration overlap.
fuel management is on you...
550cc injectors should be OK with proper tuning.
exhaust wise, for 400HP you are going to want full 3 inch turbo back exhaust, inclusing 3 inch downpipe and an external wastegate to prevent boost creep.
~boom