Are these 340 exhaust manifolds

340 manifolds have a taller exhaust port than a 318. You might have a port mismatch and a leak. Try putting on a 340 exhaust gasket on your 318 heads and pay close attention to the bottom of the port. I had the same problem on my 318 heads, big exhaust leak. I have a set of 318 manifolds for an a-body that are in good shape if you are interested. Bob