i think 400 is achevable eith that turbo (but only just)
if you can afford it, new 200 block, new standard pistons like mark said.

but dont wire ring the block, its not worth the hassel. just use standard headbolts with a cometic multi layer head gasket! is capable of holding it together at a much higher spec than you are going

if you can afford it you should be ok with those pistons, i have a mate running 400bhp on 1mm oversize and he aint had any problems, yet anyway