You asked for the "easiest way". I believe that is going to be big boost.
More cubes will do it, but it isn't always easy. A lot of work goes into building an engine properly and making it reach 600 hp without coming apart on you.
Don't take this the wrong way, but have you ever been in a 600hp car? It's not exactly a common street car. It is a lot of power.
I remember the first time I experience around 500 hp. It was in a chevy truck with a 454, aluminum heads, etc. Anyway you get the point. It had so much torque leaving the stop light that it twisted the drive shaft and threw it over 100 yards down the highway. I was so glad to get out of that truck and even more happy no one was killed from the drive shaft getting thrown. I'm not exagerating either, I mean I went and picked up that drive shaft literally 100+ yards away. The only reason I knew where it went was because I saw sparks from it bouncing off the pavement and it was twisted like a beer can.
Anyhow you can get the power easier with boost. And it can be a lot of fun, but it can also be more power than you might really want. I am more into N/A engines. I just find it more fun to try and reach a HP goal without FI, but it doesn't matter how you get it as long as your happy in the end.