400WHP or just 400CHP? (around 340-350WHP)
Other than a good built CB with forged pistons (not totally needed, but you said reliable) your going to need a good ported head with custom intake and F4 cam at bear minimal to make 400WHP @ 24psi or less. You don't need a header, but it will make it easier IF it's built right. Also going to need a 60mm turbo or larger, this will be key as most are not very good at matching turbos for max response + huge power.
The Charger is a perfect eg. of what your looking for. Going on it's 8th year without an engine tear-down, all I've done is change a few H/G's over the years.
Get's 38+mpg on the hwy and can drive it anywhere.
making around 550WHP @ 39psi boost (about 630CHP)
BTW, I've been meaning to write up a stage 3 build thread, so it would give you and others a clearer picture of going from stage to (400WHP max) to stage 3, apply named beyond +40 inj's.
If anyone is curious, I consider the Charger to be a stage 3 car. Then I can add a stage 4 (final frontier) thread to finish things off :)
Willing to bet it's making 400WHP+ @ 24psi