Good 400+ HP 360 build

Although they sounds cool, I've heard that they can actually hurt performance so I don't want to go with a thumpr cam. I have a comp 256XE cam that was initially for my 318 that crapped out, I know the cam would work, but is it too small? I guess I haven't really said my parts yet. I have a set of rebuilt J heads with 1.6 and 2.02 valves and hardened seats. Aluminum weilant 4 barrel intake manifold and a holly 650, and that 256 cam. I want to add new pistons and achieve close 9:1 compression or higher. I've been looking at other cam shafts such as the comp 262XE, but it says that I need adjustable valve train, why is that? What would happen if I run it without adjustable valve train?