If you still want 400 rwhp and it is going into a (heavy) truck,what you should consider is a bigger engine with more torque.You might need as much as 550 HP to get 400 to the rear wheels. To get 550 hp from a 355 expect to rev the engine to 7,000 rpm (ish) or you need a power adder. Any engine that makes 1.5 hp per cubic inch is a serious engine with all the right parts.
Buying a cheap core is a very small part of the project,,,you will be miles ahead just buying a new block with the appropriate size to start with. A mild 454 is a better place to start if you have little experience with engines.If you rally know your engines then build the engine that best suits your style of driving.