budget 273 build

Hello, my name is Jedidiah Rumbolz. I have a 1966 dodge dart gt with a 273 that is in need of a rebuild. I was wondering if any of you know a couple tricks to get a couple easy horses out of this engine. I am aware of a package called the D- Dart that dodge did in 1966 for racing in D/Stock. I believe it had a 273 with a .500 lift cam, 4speed, and a 8 3/4 with 4.88 gears. Will the heads on my engine flow at that lift? I only planned on taking this engine to 6000rpm. How much spring pressure can a stock valve train take?