273 hop up help..manifold carb etc!

Hey guys I have just got a 68 dart with a 273 and I have been trying to find manifolds but have come up really empty which is stumping me. I have been trying to do some research and supposedly the 318 manifolds bolt on to the 273 but not earlier 273's?? So my 68 is safe to swap a 318 onto it?? Thanks all.