I have a 77 D150 with a 440 as well. I have headers on mine and because of where the fuel tank is located, I had to run the left side pipe across and down the right side to clear the tank. I would love to run an x or h pipe in the system, but find it difficult to do so because of how the pipes are configured. You might get away with running a pipe inside of the tank when using manifolds, but unless you have a rear mounted tank, (i.e. Ramcharger style) you'll have a tough time running headers without running the left-hand pipe across like I had to.