64-66 273 exhaust manifolds will work with 340/360 heads. You need to use 340/360 gaskets. I slightly ported the top and sides of the manifolds but, do not let the sealing surface get too thin. The passenger side can be switched to a 71 up 340/360 manifold, but the "bell" has to be reformed for the bigger outlet on the exhaust pipe. Other than that they are the same , with larger passages and outlet. A good custom exhaust guy should be able to do this.