The nice part about building it with the 318, is that you put it together with the Stock 318 Exhaust Manifolds and you still get great performance with that 340 cam, they just flow well together.
Then you don't have to fight the PITA headers that are not fun to install, are in the way to get the spark plugs out, are in the way when you want to work on the car. Headers are 8 Pipes in the engine bay giving off excess engine noise and additional heat, where as the Cast Manifolds sends the excess heat and noise out to the back.
If you want more flow through the engine put 2", or 2 1/4" or 2 1/2" exhaust system on it all the way to the back.
2" will flow fine and is easiest to work with.
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