Can I get away with stock 318 exhaust manifolds on Magnum crate engine?

I have the 340 manifolds on my magnum headed 340 and the port is taller on the manifold than on the head. Since the port is not as tall on a 318 manifold, it should bolt up and seal fine.

Yes, it will choke the motor down, especially if you have a small diameter exhaust system. But the 360 crate engine will be a lot more fun than the 318.