O-ringing is somewhat expensive because it's labor intensive. Remove the head, cut a groove in it and use a different gasket and fire rings. If you keep the boost below 60psi, this shouldn't be necessary.
400hp isn't hard to accomplish, but there's a clean way and a dirty way to do it. The dirty way is obviously cheaper, and will yield more EGT. Camplate kit, relatively large injectors and a turbo (plus the standard stuff - gauges, AFE box, exhaust).
The clean way would to get some more power by going into the pump. That will cost more, plus means the truck is down while the pump is being upgraded, but then you can use smaller injectors and it will smoke less and make less EGT. Same turbo and other stuff needed. Power is power.