Easiest way to gain HP is to free up the robbed HP. do a full tune up, clean the carb and fuel system, change trans and PS fluid, oil, and Let it breath better in and out, some sort of an intake and exhaust.
Thas why I said "some sort" of an intake to get cooler air to the engine. Like run some custom duct work up to it or what ever you could come up with. But the tune up, fluids, and fuel system ceaning will certianly get you started.