to build 400 horse power you need heads that flow over 200 CFM of air on the intake side.Ideally the exhaust side flows 75% ratio of the intake side. The most common heads used on 400 HP engines flow 240 cfm. Modified vortec heads/e tec aluminum heads/dart SHP and many others.If you have good suppliers in your area,then ask them what they sell.
It is very important to match your heads to your compression ration with proper placement of piston in the block and camshaft profile.
The size of engine is up to you.The bigger the engine the easier it is to make 400 hp.IE: a 400 cube engine is easier to make 400 hp than a 305.
Decide on an engine size you will use and almost everyone can help you decide on the parts.You might want to consider the budget when you post what you want.Quality parts enhance reliability and longevity.