340 Manifold questions

From what I know and what I have been told you are not going to see much of a difference if you make the swap... if you are planning to do any racing at all the swap may be worth it because even small gains can mean a lot for ETs in a specific class, but if its only for the street I don't think you will make it into the ranges to see those gains often enough to spend the extra money. Only other reason I would change it is if you're just a purist and HAVE to have all 340 parts on your engine.