Trying to get 400hp from a 360

Basically a stock-port-headed 360 can make 400 hp if it's built well enough. If you use stock heads, to get the good work and parts, you will spend what a set of Brian's RHS heads go for, and the RHS head is better than a stock LA head. But, decide on heads before you order pistons. I would not use the flat tops with those heads. I'd use a forged dish and run a tight quench. Internally balance it, run a dual plane intake (Stealth, RPM, RPM air gap, .. whatever), a Street Avenger 670 carb, and a hydraulic cam around 230° @ .050 and you'll be there or more on pump swill.