I think you need Better items in the build to handle 400 bhp reliably.Which also means more expensive.You probably could do a 400 bhp motor for not much more than a 350 BUT you'd be asking ALOT more of the cheaper parts you put into the build which would eventually BREAK.then your back at square 1.You may be lucky and get away with it BUT i would rather do it ONCE and do it Properly.

For Example
Head Gasket.
Grp 'A' would probably be ok for 350 About £80
For 400 i would go for a Proper WRC 4 layer one £350 As your running more boost etc.

Do that kind of thing to say 5 items and see where i'm going.

HTH