I'd avoid using headers & manifold in combination as the flow rate differences might cause some balance and vibration problems if it is pronounced enough. Might also cause mix problems. Try to keep the engine running as symetrically as possible.
Maybe some of the hot-rod headers would work for you, either that or there are a tonne of aftermarket 5.0 headers which might do the job. Hard to say without seeing which would be best.
If you go with ebay specials, then at least experimenting wont cost you much