How to make Horsepower?

273 gives the best answer to his exact question of "more HP". Changing the rear-end is the answer to a different question. The basic technical answer is to keep torque up at higher rpm, since HP = torque x rpm. Torque comes from cylinder pressure, which requires maintaining a high intake pressure, which means a free-flowing intake and exhaust. Without changing the valves and head, a turbo will supply the pressure. However, that is a roll-your-own solution currently.