very curios to see your results, if you are running a cam with less lobe seperation (more overlap) the scavenge effect of the headers will probably give you quite a bit more power (50 hp does seem like a lot). with wide lobe seperation like factory cams 113 plus degrees, you might not see anything with the headers. are you running factory cast d ports? what cam? I once read about a test done with a stock 76 455, the only change they made to the engine was to put headers on in place of the factory "log style manifolds" and it made no significant difference to the engine because the stock cam was too small to make the headers work