What would I have to do to get up to 550rwhp on a 04 GT Mustang?

The easiest way to do that would be a twin turbo setup. You'll need to have forged internals as well to cope with the boost necessary to achieve 550 rwhp. You'll also be a the very limit of what the stock block can tolerate. I wouldn't expect such an engine to have a great deal of longevity. Expect to spend around $6000-$7000 all together.