Ok, if you have 340 manifolds and want to use x heads, thats how 340's came until 69. I would recommend flatout graphite manifold gaskets. Yes headers would give you more hp, the more all out the motor the more benefit. That being said I run 12.10 at 113 through stock 340 exhaust manifolds. With the bump in compression high 11's definitely in ball park. point is you can get a bunch out of the 340 manifold.
