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

Depends on if you need to do anything to the bottom end. As long as the bores, pistons, rings, bearings, etc are good, you could bolt on some good used heads, 302's or magnum heads have smaller combustion chambers, so that will give you a little more compression without changing pistons or zero decking the block. Add a decent cam, lifters, springs, pushrods, intake carb, and headers. Convert it to electronic ignition if it's not already. You might be able to make it on $1500, but it would be pushing it.
Used/rebuilt heads; $300-$600
Cam, lifters, timing chain; $250
Valve springs; $80
Intake (used); $100-$150
Carb (used); $200
Headers (used); $150-250

That puts you in the $1100-$1600 range. But keep in mind, something always comes up that you don't count on. I'm by no means an expert, but that will get you some power.

Best thing to do if you want lots of power though is to rebuild the whole engine. If you need to drive the car, then find another engine for cheap and slowly build it as time and money allows.