timing the same for headers set up? lack power?

hedders should add 10% higher HP for what ever the engine puts out, your ignition timing wont change as much as your fuel requirement will, buy a set of jetting drills and add 10% to your jetting size and start there

Its all about volumetric efficiency, hedders work with camshaft design and carburater size, valve sizing, working RPM range and on and on and on

your doing the right thing, a good engine build starts with how much spent gasses your system can handle instead of how much fuel you can cram into the engine, now that you have set how your engine can exhale, now you can work backwards to find out how much it can inhale

If you really want to learn how to do it right , look up the formula for 'pounds per hour of fuel ' and will help you understand how it all comes together