1500USD to build as much horse in a 360 as possible. What would you do?

It is a matter of debate, but I've heard others argue that there is no use making any modifications until you have dual exhaust and headers. Headers will give you more hp at all rpms with no other mods. They also increase torque. They will bite into your budget but I think that they would be money well spent. They are probably among the cheapest hp/$ changes you can make.