How much horse power will headers and a true dual exhaust add to stock 307 olds?

You won't see a dramatic horsepower increase from just headers and a true dual exhaust on a stock 307 Oldsmobile. While these modifications improve exhaust flow, the gains are relatively modest on a naturally aspirated engine that isn't heavily modified elsewhere.

Expect gains in the 5-15 horsepower range at best. It's highly dependent on the quality of the headers and exhaust system, the overall condition of the engine, and how well the rest of the system is tuned (including things like carburetor jets, timing, etc.). The lower end of that range (5-10hp) is more realistic for a completely stock engine. You'll likely notice a bigger improvement in torque than in horsepower, leading to better throttle response and acceleration.

To get truly significant horsepower gains, you'd need more extensive modifications like a performance camshaft, improved intake manifold, carburetor upgrade (or fuel injection), and possibly cylinder head porting and polishing.