How much H.P. would a set of headers give me?

Fenderwell headers can be tuff to do. Tti's would be an easier fit at a price.
On a mild build, stock engine to stock and bolt on's, a general rule of thumb we used to use was 15%.

This is also due to the old carbs/engines running rich from the factory and the headers would lean out the mix to proper (or close to proper) levels/ratio's.

A typical 340 @ 275 HP would generate 300+ HP everytime, tuning the carb just right was allways worth another 10 peak HP.