adding HP to 273 small block

The 273 is a small mill.

Make your choices on the conservative side.

If you're not into a tear down, buy small bolt ons. A big carb will kill you, so will a big cam ect.

I would start with regular bolt ons...a good dual plain manifold, headers, a carb upgrade, a good ignition system.

Then maybe a custom ground cam from Racer Brown (it only sounds expensive)...

The heads you have will meet your needs. Don (cuda66273) runs 11's with those heads (heavily modified of course, but still small valves) on a 318.