Can I use 318 exhaust manifold on 340?

There is enough difference the other way around that without gaskets you can have a tic tic tic type leak on some early 318 head castings with the 340 manifolds. The bottom of the port openings is so much bigger on the 340 that it is dead even with the outside bottom edge of the 318 exhaust port. The 318 manifold should seal but will be restrictive.